The level starts off with a Rain D. A few more Rain D will spawn, then the boss (Elder Sloth) comes. After the boss spawns, R Ost will join the spawning party, entering the mix with a spawn rate slightly slower than Rain D.
Bringing Rich Cat and Cat CPU is very highly recomended.
Start off by waiting for the first Rain D. to spawn, then spam all your meatshields and King Dragon. Slowly spawn your expensive stuff. If you have the Cat CPU, turn it on when all your expensive stuff has been spawned and your wallet is maxed.
Megaphrodite is useful for this stage, as it can hit Elder Sloth for massive damage while protected behind meatshielding for the R. Osts and Rain Ds.