I didn't do it right away. I did some missions and got a lot of Reviver Seeds, and then took them with me to the dungeon. So my recommendation is to go back and do some missions to train up and buy/find/obtain a lot of Reviver Seeds, then go back to Deep Dark Crater. Then be strategic, and you'll eventually beat it. It's a matter of persistence. Don't give up, and you'll eventually succeed.
Hope I helped!