If a DM wants to throw everything they’ve got at a great weapon master fighter or barbarian, then some of the best monsters to use are monsters with a high weapon damage resistance and a very high AC. For example, higher level undead have resistances to several types of damage as well as a high AC. Monsters with large pools of health can also work too. Additionally, you can go much smaller and have swarms of monsters as well. Swarms have a lot of damage resistance and there’s no way to reliably take them all out with one or two swings, so the feat is going to be a lot less useful.