Well, to answer your first question, I'm Fumbles16x/Beezus. I play a lot of Phantom Assassin and she has grown to be one of my favorite and most successful heroes. She is considered to be a bit of a pub stomper, which isn't inaccurate, and kind of "ez mode." She's certainly not overly complicated, but I don't think you can just walk blindly into a match and expect to do well with her. So that's what I'm here for!
Phantom Assassin or Mortred or PA, is about as straightforward as it gets in Dota 2. She kills things. Her skills are specifically designed to, as her name suggests, hunt you down and destroy you. Due to having the best critical hit in the game, she can make short work of all but the tankiest heroes, and is a formidable manfighter at all stages in the game. She excels at dealing huge amounts of damage in short periods of time, while remaining elusive herself, much like Weaver.
I will go more in depth on her abilities later on in the guide, but know this: Phantom Assassin is a great hero to learn if you're new to the game, and is still useful even at higher levels of play, meaning that if you're interested in playing carry, she is a great place to start.
1. She's forgiving. You can afford to lose the early game with this hero because she can always make a kill happen. Her crits do NOT mess around and can turn a fight around in an instant.
2. She's fun. Seriously, blinking around the map and cutting people in half is a hell of a good time. Also winning is pretty fun.
3. She's far more exciting than other carries. She doesn't have to sit in lane or farm the jungle for 25 minutes before she can fight. She's good as soon as she gets a couple of low cost items and then she can start wrecking face.
4. And more. I'll just let the guide kind of speak for the hero. Chances are you know a little about her already since you're here. I'm just going to try and help you get started with her, and give some tips to help you start getting that winrate up.
Depending on map control, items, and other factors, Mortred can solo Roshan fairly easily and quickly. Not quite as fast as Ursa, but probably around as quickly as Drow Ranger and Troll Warlord. Soloing Rosh is incredibly beneficial to you and your team, as you get a free life, a ton of extra exp, and a nice gold boost to your team.
Though it's still best to smoke in, Mortred can sneak into Rosh pretty easily with her Blur ability. It also helps you survive Roshan's attacks because of the huge evasion. As far as items go, you only really need some lifesteal and levels. By the time you have basher and lifesteal, you can't really die to him.
The strategy for a quick and easy Rosh with PA is this:
1. Smoke into the pit. Make sure wards are up so that you can see anyone who might try to gank you inside the pit.
2. Use Stifling Dagger to pop Roshan's spell block, then immediately Phantom Strike him for the boosted attack speed. Attack speed means more crits and bashes, which means a quicker Rosh.
3. Repeat. You can usually use Phantom Strike 3 times before having to pop his spell block again with dagger, but that should be plenty to kill him anyway.
Make sure you always have a slot for the Aegis. PA doesn't really have slot issues until the really late game, so you should be able to pack an Aegis most of the match. Try and keep an eye out for Rosh as often as you can, because it gives your team a HUGE advantage. He respawns every 8-11 minutes and the Aegis lasts 5 minutes. Always be aware of these times so that you know how long you have to push your advantage, and how long you have until you can go back into the pit.
Every match in Dota 2 is different, of course, but I wanted to give a sort of guideline for what you should be doing at certain times during the game so that you know you're on the right track.
Phantom Assassin is a high risk, high reward hero. You blink on top of enemies just hoping for a big crit to put them away, but when it works it can give your team an enormous advantage and send your enemies fleeing. When playing this hero, you can never let up. You will be surprised how easy it is for this hero to get kills and start snowballing. Even in lane, all you need is a little setup from a support to start making things happen. Once you have your crit, all you need is a little RNG to get a kill, honestly.
It's situational. If you have an Enigma or someone who can pack up the enemy team in a nice, tight cluster, then a bfury can be decent. It does help farming speed, but.. it takes 15-20 mins to build it doing nothing but farming, and you're too good at fighting to waste this time. You come online faster than almost any carry other than perhaps Troll Warlord, so you need to just embrace what you're good at. Go fight people. Farm heroes. Kill their carry while he attempts to farm and cause a major gold swing. More reliable gold (from hero kills and buildings) is better than more unreliable gold (from creeps), so it's best to just get pickoffs and push towers. The more active you are, the more likely you are to win, and Battlefury is not the item for that playstyle. You are not Anti-Mage.
Like all heroes in Dota 2, Phantom Assassin shares a certain amount of synergy with some heroes, and is countered hard by others.