Kingdom Rush Frontiers

I did it on the Beresad's Lair level, just put 4 or more magic towers and level them up to level 4 and choose the upgrade on the left. The skeletons come out on their own.

>Necropolis - Achievement Video Guide<
Level: 12 The Underpass (Heroic Challenge) because you start with 3000 gold, there is only one exit and all upgrades are available.
Hero: Cronan with upgraded Boarmaster ability. (recommended)
Hero: Kahz is not recommended unless you disable his Bull Rush ability because it's making him running rogue all over the map.
Upgrades: Reinforcements (max), Soldiers Barracks (max), Mage Towers (max), Rain of Fire (max).
Build 2 Necromancer Towers - 800 gold each. (without any upgrades)
Build 3 Level 3 Knights Barracks:
-1st rally point will be next to the hero.
-2nd rally from the top will be as low as possible on the path.
-3rd rally from the bottom will be as high as possible on the path.
Build additional Necromancer Towers in the middle of the map as the level progress.
Don't call waves early.
Use Rain of Fire when enemies get close to the exit.
Use Rain of Fire when bigger enemies get close to each other while fighting your infantry.
Use Reinforcements to kill the flying enemies on waves 2 and 3.
Shouldn't last longer than 3 waves.