Punch Drunk

Defeat twenty opponents while a party character is drunk in a single playthrough.

This achievement can be accomplished anywhere in combat by any character who imbibes alcohol (immediately before or) during combat and then, having damaged a particular foe in combat, kills that foe. The important thing to remember is to drink before dealing the lethal damage.

The easiest all-at-once method might be in Act 2. Near the area where you met the Harpers and the shadows, you will find an old building and a clickable "Raven" along side the road ( at X:29 Y:118 ). When you click the raven, you suddenly find yourself in combat and get attacked by many birds which only have one hit point. You can use one Ithbank (any alcohol will work), give the drink to Gale and have him consume it. Wait until the ravens encircle the party, then cast shatter on them all. No more angry birds. Achievement complete. This strategy also worked with Wyll casting "Shatter", although any good AoE such as Shadowheart's Spirit Guardians should work.

This could also be accomplished during the Roveer's Storehouse sidequest in Act 3, where there are multiple rats to kill that, like the ravens, only have one hit point.