Tips for Level 18
Don’t even defend from the outside, use barricades to block all the stairs leading to the rift except for one, and then setup a killbox somewhere on the path, preferably in the spot with a lots of ceilling space.
I myself used tar traps combined with lasers and arcane dragons. Once you reach the wave where enemy start to come from the back, use barricades once again to redirect them on the path with your killbox.
With all the stairways blocked by the barricades, any additional rift enemies spawning in will have to take the path you’ve created as well, as no additional rift can open underneath the stairs.
This approach makes the first two waves the hardest, as you will have to invest a lot into the barriers, so you will have to kill some passing enemies by yourself, but after like 3rd wave, there shouldn’t be a situation where you will be in danger of losing, if you just keep expanding on your killbox.
You can also achieve 5 skulls this way with ease, personally I was even rushing it a lot, starting waves immediately and going in to kill some of the enemy waves all by myself, and I ended with more than 8 minutes to spare.
The last level was actually the easiest war scenario, because it is the only war level without a boss encounter.