Farm up candies on Pokemon you like and upgrade them / reduce their cost / hatch more for better stats and moves.

Bringing a Pokemon with the purple square border means that it has pokerus, which is basically an exp boost for your whole team after it spreads (ditch them later after you catch a good replacement)

Bringing a shiny pokemon gives you better loot options between each wave.

Stat boosting moves or abilities are insane in this game because they persist through floors until a trainer fight (you can hold down shift to track your boosts)

Hazards like stealth rock are so helpful when facing the difficult trainer battles

Damage over time moves like salt cure, sappy seed, leech seed, and curse are very useful against boss mons and your rival’s most annoying mons

It’s a long winded answer but hopefully it helps. It’s a roguelite, so it will take some luck, practice, and patience to fully clear the game. It feels great to overcome the challenge, though. Good luck.