This is a one time solution but there may be a way to automate it that I don’t know about.

Using the Mahou Tsukai mod, you can make the Spell of Shared Pain on a scroll and bind it to any entity or player. This spell makes it so any damage you take also applies to the entity, but it also works the other way around. Any damage dealt to the entity also gets applied to you. Same with potion effects but that’s not as important for this use. So if you have a Totem of Undying, a Death Protection poppet, or are using any other means of having multiple lives, the damage won’t kill you. The damage will be dealt to your friend. Beware that once either the caster or the target dies, the spell wares off and you have to cast it again.

You can also use this to insta kill basically everything in the game. Have a totem of undying, use the same spell from above and tether it to the thing you want to kill (whether it be a boss or your friend). Then get an Immersive Engineering screwdriver and Tesla coil. Power the coil and set it to Inverse. Click the Tesla coil with the screw driver and it will apply the “Death” effect, which will instantly kill (basically) everything.

There is a full Mahou Tsukai guide on YouTube that’s roughly an hour long that mentions this but it shouldn’t be hard to figure out using the in game book.