| Wave clear | Deletes groups of regular enemies before they reach the base. | Wide coverage or multihit attacks, an attack cycle that does not stall, and upgrades that stay affordable through the mid-waves. | Normal enemies leak on repeated runs while your boss damage is idle — your clear unit is under-leveled, misplaced, or outscaled. |
| Boss damage | Melts the single high-health boss health bar before its timer or push ends the run. | Sustained single-target output, enough range to stay on the boss, and an upgrade path you can afford during the run, not after it. | Waves are fine but the boss walks through — add or deepen a dedicated single-target slot. |
| Support / control | Slows, stuns, buffs, or otherwise makes the two damage roles better than a third damage unit would be. | An effect that scales with the rest of your lineup. One good support usually beats two mediocre ones. | Half your filled slots are support and runs still fail the damage checks — cut back to one. |
| Economy | Improves your income or roll flow so the team comes online earlier in the run. | A visible income effect early in the run. If the boost does not change when you can afford your next upgrade, the slot is wasted. | Your runs are lost to damage checks, not to being broke — trade the slot for damage. |
| Flex | The sixth-slot experiment: a second boss unit, a map-specific answer, or a unit you are testing for a future promotion. | Whatever the current map punishes you for lacking. | It has not earned its slot in three consecutive runs — promote the bench unit you have been meaning to test. |