LEVEL 1 - Mist´s range is very important
LEVEL 4 - This is why u can be a assassin, very good talent ;)
LEVEL 7 - But u are support too, so u must heal everytime
LEVEL 10 - U can teleport away and heal everybody. But second talent is not bad too :)
LEVEL 13 - Slow the enemies, then polymorph - KILL THEM!
LEVEL 16 - Safe your enemies from dead! Everybody!
LEVEL 20 - you are HEALER, so heal! ;)

Brightwing is very good support, but if u know, how to play him, u can be good assassin. OR "line defender" .. Good in teamfights, but he is not easy to play.