Warlocks can be incredible for offense: eldritch blast + agonizing and repelling blast invocations with high charisma is a lot of at-will damage. Pact of the blade lets you ignore physical stats and be in melee with any weapon. fiend patron is great as it gives you temp hp on every kill and access to fireball. My MC is a full warlock and consistently kills half the things in a fight. Repelling blast in particular is incredible if you keep an eye out for enemies near ledges. With good positioning you can be booping two enemies to death per action.