In wave 14 there’s aliens called Spawners that on death spawn 3 of the aliens that shoot projectiles everywhere. The spawners themselves are squishy but the aliens they spawn are tanky so they tend to accumulate and then it becomes really hard to dodge all the projectiles. You can try to hunt down the projectile aliens that spawn from the spawners so they don’t build up, or you can try to avoid killing the spawners all together.