I got this playing a custom match vs Most Gentle AI Drowning Doom on Feeding Area.
First, capture every geyser on the map. This includes the one near the enemy stage, to slow the AI down while you prepare. Upgrade your stage all the way while you repeatedly kill Ophelia and anything she summons, then purchase a Bleeding Death.
You may also want to set up some squads guarding the other geysers. I found Warfathers and Skull Rakers work well for this.
Now stand/fly just outside the enemy stage area and play the waiting game. Let Ophelia summon Grave Diggers and wreck/replace your merch booth right next to her stage. If she summons a Bride or Frightwig, kill it, then fly out of there. She should eventually have 3 squads of Grave Diggers, making 18 enemies. (If she sends them into action before this, this is what your guard squads are for.) Once you see the 3rd Grave Digger squad emerge from the stage, play the Launch of Death solo to send in your Bleeding Death. While your monstrosity emerges, run in and get the enemies' attention and draw them back towards your killing machine. After pounding the Grave Diggers into the ground, the Bleeding Death should go after the stage. Ophelia should summon some new Grave Diggers during the rampage, which will make up the other 2 you need for the achevement.
This technique also works well for Dollpocalypse, Ringleader, and Euthanasia.