In early Act 1, use
Explosive Grenade to clear trash monsters and kill bosses.
Flash Grenade is used to help clear trash monsters, and stun bosses. If your gear is strong enough, flash grenade can one shot entire packs.
At Level 14, your trash-clearing rotation becomes: throw
Gas Grenade, then
Explosive Grenade to detonate it. Throw them ahead and run past so mobs follow into the blasts.
For bosses, cast
Gas Grenade, then
Explosive Grenade, and continue throwing gas / explosive grenades on cooldown. Gas Grenade has a limit of 6, so before you start using
Bombard Crossbow, you can cast
Gas Grenade twice per
Explosive Grenade (rather than once). Once you get
Oil Grenade, you should throw one out before continuing your usual rotation.
From level 52 for clear, cast
Cluster Grenade, and you will one shot the screen.