Kael is one of the best first picks in the game. He has a lot of damage, crowd control, can push and most of all can adapt.

Kael is a valuable pick, hopefully 1st but if you can you pick him whenever!

Gravity burst is the go-to build for Kael thas. It is mana efficent, has high damage burst, can clear minion waves with 2 spells and even throwing spells everywhere still gets value. But it has one weakness: tanks. If the enemies have many high health heroes like chogall, azmodan or any tanks it can be tricky to kill them (yes I have tried flamestrike builds but it is trickier to catch 2+ heores in 1 explosion than it is worth it). For that reason I use infinite bomber as a backup.

Infinite bomber has a much more conserved playstyle than Gravity burst's "shoot everything" playstyle. Flamestrike becomes a waste of mana, Gravity lapse is a last resort escape and Verdant spere-Living bomb combo should be spammed for damage, wave clear and mana regen (trust me no mana addict makes you really mana hungry). After level 16 you can easily set 4 bombs off before even the 1st one explodes, so after level 20 your enemies must remain spread all the time and they still take a lot of damage. the downside with this build is that it's harder to burst someone down since you have to use unbuffed gravity lapse and flamestrike for damage (and that burns ~35% of your total mana per burst!).