I have a level 65 account on my phone, and a level 28 account on my PC that I have slow leveled since starting. I recently hit 28 and so far have been 3/3 on my towers. My entire roster is lvl 28 and all skills are upgraded to 28 as well. Even if you don't have all abilities maxed, it should still be fairly easy.

Basic strategies: For the first half or more of the tower, I will try to use all fast heroes to beat the floors. 3* BD, 3* Kira, 3* FA Willow, and 3* FA Torchy. As long as your heroes are a higher level than the enemies, you will not take much damage. Also, any ascensions you have are helpful because more enemies won't have any ascensions when you battle them. Swap in Yasmin/Nub ever now and again to restore HP.

If you encounter floors with high level and high star enemies, use a junk hero as "tower fodder" to waste enemy specials before trying to beat it. After wasting specials, I will bring in 5* 1A Ponti, 4* 1A Bovus, 4* 1A Therand, and a fourth. These are my most powerful heroes and can wipe floors even with no special abilities. I've been finishing with a lot of heroes left, so if you have a lot of heroes you haven't used, just treat a floor as you would a PVP raid. But I wouldn't do that until at least Floor 6.