Well, for melee, look no further than Callous Execution (in the Gruesome tree). I would be useless against bosses without this skill. I've modded it to cause bleed damage, to stun and to crit more often. I keep it at a low regen rate and use plenty of -chance for monster to evade items so that it lands as much as possible. I also try and maintain a 40-50% deathblow, one of the most powerful item mods I've personally found.
Ruthless Mutilation is also a great skill to hit multiple enemies at once.
In the Astute Supremacy tree, you have Clustering Maelstrom which is essentially a black hole that pulls enemies inside it. Modded properly, it can be used to kill massive groups of enemies at once.
My trick is to use the Astute tree for multiples, and for bosses / tough champs, I use Callous Execution along with Dislodged Spirit (debuff) and also frenetic fervor (haste, chance to hit, etc).
Inquisitors also have great combo potentials - you can combo Clustering Maelstrom and Levin array together for example; this will cluster the group of enemies, damage them and apply levin array to them as well.
For a debuff, if you mod mortifying pillory to reduce enemy defense and rune it high enough, it combos really well with dislodged spirit.