After you've unlocked Endless Mode, make a mech loadout with maxed health, a handful of extra jumps, and maxed secret detection.
Start an endless run and be extra vigilant for secret rooms. If you're like me, you might not know the "giant purple rooms like covenant from Halo" even have any secrets, but i guarantee you they are there, just much harder to see compared to the long yellow rectangle secrets from the normal rooms.
Don't forget to kit yourself out to survive, if you get screwed over with a few 0 secret rooms, you'll be up against the wall trying to clear the heavier rooms trying to find the last few secrets.