Yes , its super hard. I reached to lvl 20 without problem but up to lvl 25 it got harder and harder. Now im stuck on lvl 27, lost 3 times and my roster is severely wounded. I havent used items besides mana and heal pots because ive used most of my items on grimforest.
I think with tornadoes and some carpet bombing the difficulty soft a little bit.
Anyways i need to keep improving. Maybe with the new costumes that will come i can grow new powerful duplicated heroes to enchance my roster and have more oportunities without invest in more 5s☆. Not sure about spaming items yet, though the loot and emblems get better after lvl 25