The level you should use for this achievement is the first mountain level (3-1).

You should use this level over 3-3 (which is more close to how you'll be playing this level), because the ranged enemies won't kill themselves on your buildings like the bombers will.

The first part of this stage you want to focus on killing the first enemies but without a strong enough setup to last the whole level. This means ideally a couple rooms protecting your hotel on the sides and a stack of guns on top (2-3) which are all connected and hopefully will die together before they finish off the enemies. You want there to be around 5-7 ranged enemies left at the end. And you should start with some healing rooms below your hotel. You want 4-5 depending on how many enemies there are. Add rooms as needed to prevent the hotel from being destroyed, but it's fine if it takes damage since it has more health than a normal room and is easier for your healing rooms to keep alive.

Be sure you don't add too many healing rooms too quickly, you still need those guns to die. If they die too early, you can replace them.

Your second goal, (once the guns fall) is to make this self sustaining. An infinite loop where they can't kill your hotel, BUT you don't want them frozen that often. You might need to add 1-2 freezing guns to slow them down a bit for the healing to keep up, but keep the freeze guns here to a minimum.

Goal three. A second tower. Using normal rooms under ground move a significant distance from the original hotel. The freeze towers actually have limited range. So your goal is to make a second tower far enough from the orginal hotel, where the freeze guns can SEE the enemies and shoot them, but not close enough the shots will hit. I've included a screenshot with the number of rooms they need to be away. You might need less or more depending. Add a frost gun to test to be sure you're far enough. If the shot explodes before it reaches an enemy, you're good.

Finally, just continue to tack on more and more freeze rooms to the second tower. You should get the achievement in under an hour so long as the enemies aren't all frozen, since if the enemies are actually hit by the frost shots, the guns will stop shooting.

The best part: you can walk away without needing to watch it.

The achievement will pop without you needing to finish the level. (but if you want to, just add guns)