How to use Ember Spirit
GO MID!, you're a squishy hero and you're level dependent, and going mid is the best way to take care of that.
If you're going to initiate a team fight, start first with slight of fist then fire remnant your way in there!(If you have BKB, activate it before using fire remnant). Activate your flame guard and start bashing on the disablers while your flame guard soaks up some magic damage.
Ember spirit is a great chaser if you use him right, simply fire remnant to the hero you're chasing, pop searing chains and flame guard, and you'll have a guaranteed kill.