The first Rain D. spawns ~3.3 seconds in. Hackey joins in ~6.6 seconds later. The second Rain D. follows ~24.3 seconds later. The third spawning in at 30 seconds after the previous. The fourth Rain D. spawns ~62.6 seconds after. The fifth Rain D. spawns as soon as the base's health drops to 99%.
This level is not easy, but if you know what you do, you'll have no problem. There are 2 ways to defeat the level, one fast and precise, the other slow and steady; for the first way, to make it simple, you just need to meatshield, make a bunch of Rain D's stack and time Awakened Bahamut to kill them, hitting Hackey as well, killing him, but this strategy is hard to pull off (credits to Nurse Wuffa). If you have Gothic Mitama, this stage will become very easy. Just stall the Rain D.s with Manic Eraser until you have enough money to spawn Mitama. She will knockback the Rain D.s behind the base and kill Hackey. Then, spawn A. Bahamut to finish the base (while meatshielding). However, Mitama must NOT be shot by the Hackey as she will be one-shot killed. (Only a Lv38+ Gothic Mitama can take a hit and live).
Strategy 1 (Wuffa)
Lineup: Cat-combos that will put your wallet startup at lvl 3+ & Base health over 108K (if needed), Crazed Cat (Manic may be too fast), Crazed Tank (DO NOT USE IF HE CANNOT SURVIVE A HIT FROM RAIN D.), Eraser Cat, A. Bahamut and a heavy damage dealer that outranges Rain D and attacks fast (such as Ururun Cat).
Only try this if you have good timing, your cat base must have at least 108K health to survive 2 Hackey hits, or 140K health to survive 2 Hackey hits and 2 Rain D hits to be on the safe side.
When the battle starts, stall Rain D however you can using as few meatshields as possible (Eraser is enough to infinitely stall one). When the second Rain D reaches your meatshields, stop spawning everything. When the Rain Ds are about to hit your base, send A Bahamut (crucial if you don’t have 140K base health to take the hits). If you have >140K base health, you can let them hit the base once, giving you a wider margin for the timing, but don’t count on it. Immediately after spawning A Bahamut, send 3 Crazed Cats in rapid succession (1 right after the other when it recharges), then pause. Right when the 3rd Crazed Cat dies, send in a 4th. The Rain Ds should die soon after. Then, send your heavy attacker (timing it so that it’s not hit by Hackey) and wait for the win.
Moneko, Neneko and Ramen are parts of combos.
Your main attackers should be level 40+.
What you have to do is to simply spam meatshields (remember to alternate Cat and Gold Cat so they don't overlap!) and vaulters, adding Paris when you have the money; try to time at least the vaulters so they don't die to Hackey. You should be able to stop the progress of the Rain D's without letting them advance and slowly deal damage to Hackey, eventually killing him. Money may be a problem, so rich cat may help.