Getting Endless Mode Achievements
Just go for a powerful build. Giant Rats fall off in later levels anyway because they’re capped to 198 extra damage from the relic.
Blood Prince is the strongest build if you get lucky enough to go for it relatively early. Blood Prince (As many as you can get, shapeshifters and Reflection Pearls are your friend) + Blood Magic + Fireball Secret is ridiculously strong.
I just aimed for this build and managed to get to 66 of both with a single ring (extra epic and legendary chance) by Wave 29.
Do note that Blood Magic and Fireball Secret are only available from Legendary Treasures or after Wave 14. Devil’s Rice has a decent chance of providing you and early Blood Prince.