As with many achievements in this game, progress is reset when you return to the dashboard, so you must meet the entire requirement in 1 play session, but not necessarily one dungeon.
The safest way to do this is to start up a 1-hour dungeon (You can change the dungeon timer by accessing the upper right kiosk in the main hub). Use a 2nd controller for player 2.
Play until you come across a level with a bottomless pit. They are usually blue glowing openings in the floor, similar to ice, but you fall through!
You have to push 25 enemies into these pits using shield bash (
) or charge (
). The achievement unlocks as soon as the 25th enemy is defeated in this way, so you can end the run after completion.
Not necessary, but before a run you may draw the enemy modifier "trash mobs." This makes all enemies rats or chickens. The layout "hole-ish" adds lots of pits. Both can make this a lot quicker.
You may want to play shorter 5-8 minute runs to also work on unlocking cards. I had the game crash to the dashboard 3 times in about 15 runs, always as the game was trying to load a boss room, so beware.