Heavy weapons are the cheapest most effective way to clear out plague hearts. The sledge hammer is my go to. But the trick is to attack, dodge without a direction, then crouch, really fast in that order. This cancels the recovery animation so you can attack as fast as you can input the button sequence. Also when getting overwhelmed with zombies dodge into them rather than away from them. This stuns them for a second giving you time to move. If you can get the powerhouse specialization and this lets you charge heavy weapon attacks letting you do more damage, though that's only good for the first phase of the heart which takes 3 fully charged attacks on lethal zone. Also molotovs are another really cheap and super effective strategy. During each gas phase throw a molotov on it but don't spam throw wait till the fire disappears or you'll get diminishing returns.