I'm a fan of using Sentinel to activate Hunter's Mark and Colossus Slayer. 3 levels of Fighter can add Riposte, Action Surge and a Fighting Style to this combo.

Essentially, as long as you have an ally in melee, your opponent can either target them and activate Sentinel or target you and risk activating Riposte (which Multiattack Defense greatly helps, but that depends on the level you plan to reach), and because of Hunter's Mark/Colossus Slayer, each of those bonus hits deal considerable damage.

This combos especially well with a Wolf Totem Barbarian, since you will both be reasonably tanky and they give you advantage, and any sort of bonus on a hit, like a +X weapon or a Flametongue weapon. You also have Healing Spirit to keep you both in good health after every fight.

I think this would work better with a shield and Dueling, but GWM should be fine too.