Rocket Barrage deals massive magic damage up close, so use it on single targets.

Flak Cannon makes your attacks hit all enemies in a radius, so use it in teamfights.

Homing Missile chases and stuns a target, so use it to secure kills and lock down.

Call Down deals AoE damage and slows, so use it to initiate teamfights.

Gyro deals mixed damage, so itemize against either and he still shreds you.

Flak Cannon also helps you farm, so use it to clear camps and waves.