This CAN be done with just two players, although not the most ideal way to get the achievement, it still works, you just need to make sure you leave healing available for the bot carrying the third tome to switch out when required. You don't both need to jump into the cauldron at the same time e.g player 1 can jump into cauldron first with one tome and one grimoire whilst player 2 fends off the hordes with the bots then revive player 1 when they respawn. Once respawned, Player 2 jumps in with the second tome and grimoire whilst player 1 fends off the horde with the bots. Once player 2 has respawned, you must have the bot with the third tome switch it out for some nearby healing, allowing a player to pick the third tome up and sacrifice it along with themselves into the cauldron for the final time.
Yes this method, compared to a full party method, is both riskier and more drawn out but my post is purely giving another method with a minimum of two.
Video of method (not my video)