Lina is one of the most iconic heroes in Dota 2. She is a very strong laner, team fighter, and ganker who can burst down targets extremely quickly. Although she can be played as a support, I think this is a sub-optimal role for her. Her skill set, item dependency, need for levels, and ability to scale into the late game surprisingly well for a nuker all point to her lane and role as being mid and semi-carry (position 2). I recommend picking Lina if your team is lacking burst damage and you already have a strong initiator or tank and if the enemy doesn't have Nyx Assassin, which is her only hard counter, IMO.
Lina's mid win rate isn't very impressive at the moment (46%), but I think this is more due to her being played incorrectly than her simply being a weak hero or a bad meta for her. For example, many people would point to Infused Raindrops as the reason for her low win rate. Yet if you look at her win rates by skill tier, you see that her win rate is actually highest in 5k+, where Raindrops are most abundant. Moreover, in 7.01 she has some great talents that fix her mana issues, keep her at a safer distance in team fights, and buff her attack damage and movement speed in the late game.
Rather, I think her low win rate is more due to players making one or both of these mistakes: 1) Playing her too passively in the early and mid game. She is a momentum hero and needs to keep getting towers and kills to stay relevant. Your ult should be on cooldown as much as possible. 2) Trying to initate team fights with her. As tempting as it is to burst down a hero right at the start of a team fight, you'll probably get killed shortly thereafter. She should wait for someone else's initiation and then, ideally, keep her distance while she unleashes her fury.
Dragon Slave is one of the best AOE nukes in Dota and you will always want to max it out ASAP. In lane you will use it to farm and harass. Later you'll use it for farming and pushing and nuking down enemies in team fights. Has a very reasonable mana cost for the amount of damage it does, nevertheless it is still mana-intensive in lane and you'll want to use it wisely, such as both securing a last hit and harassing the enemy mid at the same time.
LIGHT STRIKE ARRAY
Light Strike Array is powerful AOE nuke that can also be used to destroy trees. The big downside is its impact delay of 0.5s, which means that the enemy can potentially dodge it. In lane this makes it tricky to use effectively (usually you will wait for the enemy hero to go in for a last hit). Once we get Eul's Scepter of Divinity, setting up the enemy for a stun will be quite simple. Normally this skill is maxed out after Dragon Slave.
Again, your Fiery Soul passive is what lets you scale into the late game better than most nukers. At level four and with three stacks of Fiery Soul, you'll have an extra 24% move speed and a whopping extra 255 attack speed. This is always maxed last because your nukes are the more potent part of your repertoire in the early and mid game.
Laguna Blade is the largest guaranteed nuke in the game with the Aghanim's Scepter upgrade (since it becomes pure and pierces spell immunity), but comes at a massive mana cost. You will typically use this spell as soon as possible when team fighting or ganking when you're certain you have the mana for it.
Do not add your first skill point until the starting rune because Light Strike Array can be used to secure it or even get a kill. Otherwise, put your first point in Dragon Slave so that you can use it to last hit and harass early on in lane.
The talent selections listed at the top are ideal, but some are still situational. Let me describe those situations. Note that you can see the win rates of each talent here.
Level 10: I think the +250 mana is a no-brainer compared to +20 attack damage. Your combo with Eul's is so expensive (1120 mana at level 18) that having the equivalent of an extra Energy Booster (that doesn't take up an inventory slot!) is incredibly useful.
Level 15: Another no-brainer. The additional cast range will let you stay out of harm's way, which means you're less likely to die. And isn't the goal to not die, rather than to die and come back more quickly? Therefore, the -30s respawn time seems like a strange as well as pessimistic option.
Level 20: If you're concerned about getting in harm's way in team fights, go with the attack range so that you can be further from danger. This talent is also great for pushing high ground. Otherwise, the +6% spell amplification is very nice.
Level 25: A cooldown reduction on Dragon Slave might seem powerful, but keep in mind that in the late game (which is when you'll usually get level 25), the impact of your spells has fallen off quite a bit and most of your damage output, as well as your ability to destroy barracks, will come from your right click. Therefore, I think the extra Fiery Soul is usually better. If, however, you find yourself dying extremely easily in team fights and you did not take the attack range talenet at level 20, then the cooldown reduction is safer.
Let me get this out of the way right now: Even though this guide is for position 2 Lina, never shy away from buying Observer and Sentry Wards if your supports aren't doing their job. Wards are not that expensive the value they provide is critical to your success.
The typical mid starting items for an intelligence hero are Null Talisman, Faerie Fire, and Iron Branch. Lina benefits tremendously from a starting Clarity, however, since it lets her continue to last hit and harass with Dragon Slave as necessary until her Bottle arrives. Since Clarity is cheaper than Faerie Fire, this will allow us to purchase an extra Iron Branch as well.
EARLY GAME: LANING PHASE (0 - 10 minutes)
The main two items you want to get as soon as possible are Bottle and Boots of Speed. I would also recommend bringing out a TP with your Bottle for reasons I'll discuss in the section on The Laning Phase. Against a Zeus, Phantom Assassin, or some other hero that spams spells in lane, you'll also want an early Magic Stick. Infused Raindrops is also a good investment if you want to score a kill on an opposing mid caster, because you'll be able to survive their burst but not vice versa.
EARLY GAME: POST-LANING PHASE (10 - 15 minutes)
It is absolutely critical that you get kills or take towers during this phase, otherwise you could lose your momentum and become irrelevant. In order to push quickly or get kills you'll need lots of mana. For that reason, picking up an Energy Booster followed by a Magic Wand is very important. The Wand isn't critical but it is useful often enough that you should default to buying it if you're uncertain.
After that, your next goal is the very important Eul's Scepter of Divinity. You won't have slots to hold all of the components, so I would recommend keeping the Staff of Wizardry active and the Wind Lace and recipe in your backpack. That way you have the option of completing the Eul's at a side shop (by purchasing Void Stone).
The active of Eul's Scepter of Divinity, Cyclone, is what you will use to set up for your combo. Upon reaching an enemy, you will 1) Cyclone them, 2) drop Light Strike Array on them less than a half second before the Cyclone ends, 3) target them with Laguna Blade, 4) target them with Dragon Slave, 5) right-click them if they aren't already dead.
Timing the Light Strike Array is the trickiest part of the combo and will undoubtedly require practice in a demo game if you've never done it before. You should also move close to the enemy while the Cyclone is active, unless doing so would put you in danger. Otherwise, if the combo doesn't kill them and they begin to run away, you might not be close enough to get off enough right clicks to kill them.
Eul's isn't just about the combo setup. The mana regen is critical to your sustainability. The movement speed lets you gank and escape more easily. But most importantly, you can cyclone yourself to become invulnerable for its duration (therefore dodging the damage and effects of many spells) or purge off debuffs like silence. You can see then how this isn't just an offensive item but a very defensive one that is useful in every situation.
Warning: One of the most embarrassing mistakes you can make as a Lina player is to Cyclone an enemy who is already slowed or disabled. Do NOT get into the habit of mindlessly using Cyclone on an enemy before unleashing your nukes.
MID GAME (15-35 minutes)
Next you should finish Aether Lens, hopefully by 21:00. This has a lot of obvious perks but my favorite is the cast range because it affects your item cast range as well. This is invaluable when ganking because you can often times initiate before the enemy has time to react. In team fights, the cast range will let you stay further from danger.
I would recommend buying the Aether Lens recipe first and storing it in your backpack, then you have the option of buying the remaining Ring of Health at a side shop. You do not need to buy an Energy Booster --- you already have one. Simply disassemble your Arcane Boots when you are ready.
"Wait, so now I'm just stuck with Brown Boots?" Yes, and it's actually not a bad thing. You're already decently fast due to the movement speed increase from Eul's. Nevertheless, I wouldn't fault someone for investing their next 840 gold in Phase Boots to give you that extra edge.
By now you will have three core items (Boots, Lens, Eul's) and three small items (Null, Bottle, Wand). If you don't want your TP scroll in your backpack then you'll need to sell one of the small items. I would recommend starting with the Bottle unless your team is in control of the river and you'll have the chance to store non-bounty runes, in which case, selling the Null first is fine. The Magic Wand brings so much value to team fights that I always sell it last.
I usually get Aghanim's Scepter next, which makes your ult pierce spell immunity and changes the damage to pure. There are few games where this isn't relevant, not to mention that the stats, health, and mana increase is always beneficial. Nevertheless, on rare occasion you might need to first pick up a more defensive item like Linken's Sphere, Force Staff, or Ghost Scepter.
If all is going well then you should have Aghanim's by 30:00. If you don't need to save for buyback at this point, you can start thinking about your late game items or some other filler items to round out your repertoire:
Lina is a momentum hero. She is one of the strongest early game heroes and she needs to keep pressing her advantage in the early and mid game so that you can either end early or enter late game with much stronger carries than the enemy.
The laning phase is definitely the most important part of the game for you. If you're not a strong mid player, then you will need to become one before you can have an impact in games with Lina. I recommend these resources for both general mid strategy and mid Lina:
Since Lina has no escape mechanism, she can struggle against heroes who can quickly get in her face and disable her, burn her mana, or silence her. Nyx Assassin is particular nasty. She might also struggle against heroes that are hard to get close to ( Sniper) and heroes that have good escape mechanisms.
On the positive side, other than Nyx Assassin, I wouldn't say she has any hard counters that automatically make me wish I wasn't Lina. (Which is not to say that Nyx himself isn't easily countered.) In the mid lane, the only really annoying matchup is Arc Warden, but the solution is to just play it safe, keep the wave near your tower, and patiently get as much CS and experience as you can.
You can use this lack of hard counters to your benefit during the drafting phase. Whereas most carries ought to be picked last or second to last, it's really not a big deal if Lina is picked first or second. Practically, this means I don't have to lose gold as my teammates try and figure out what they want to do.
As stated in the introduction, what should concern is not so much who is on the enemy team but who is on your team. If you have no one on your team to initiate or tank, then Lina could possibly be a disastrous pick. Granted, they might not have an initiator either, but if they do and you don't, team fight could be a disaster.
Valve labels Lina as a support and a carry. That doesn't mean she can play both equally well. Indeed, Sven, Wraith King, and Viper can play support roles as well. But, like Lina, their support style is either gimmicky or very situational.
A support usually has two traits: