This guide is from my experience in getting this achievement and shows some tricks I found while grinding it that I didnt see anywhere online.
How to Obtain Captain Smushed Achievement
The whole point of the achievement is to crush mithrix with a supply beacon, its for preference for whatever one you want but in my opinion the best one to use would be the hacking beacons purely because its more useful then the other three.
The most important part of this run would be the items you pick up since they directly affect the damage and effects you apply to the boss, some items you should avoid and others are must picks for this achievement.
The most important equipment here is definitely effigy of grief. It holds mithrix in place so you can easily hit him with the beacon instead of desperately trying to make him walk into it, combine this with fuel cells and you should be fine.
A power elixir is very helpful to have, when damaging mithrix in his dying form, each attack does alot more damage then the last and its hard to judge when you can kill him and when he can take another hit, the power elixir is funnily enough for mithrix to use when he takes your items, not you. Theres the chance this doesnt work but it saved my run.
Shaped glass and armour-piercing rounds can also be helpful as you dont have to get him too low, just make sure you’re aware of your items and when you grab them because the double damage primary fire could end the run.
Singularity band and tentabauble are other ones that can prove to be helpful, although not as consistent as effigy its still helpful
Any items that cause outside damage like tri-tip dagger or atg missile can very very easily kill mithrix stealing your kill before the beacon can land. Its better to just ignore any items that can possibly kill him before it will land.
Gesture of the drowned is unnecessary with effigy due to its infinite lasting effect and short cooldown, picking up one of these will just cause chaos for yourself so its best to avoid.
If you have a dios theres a chance you can use your two shots and he’ll get revived. Im not sure if this still gets you the achievement since you technically did kill him but better safe then sorry.
Very importantly dont pick up any drones whatsoever or spare drone part, this type of damage is worse then atg missile or tri tip dagger. Stick to healing drones for this run.
PS – If you have friends (improbable) they can help by playing characters that have abilities to stun mithrix or wittle him down slowly for you, for example railgunner and artificer can freeze mithrix in place for you.
The fight is pretty much a normal run, just obviously save your beacons for the very end.
When you get to the last part make sure your very very careful about what items you get back and his health. Your damage can double after a slight hit on him so just be aware of that.
When he is at around 2k health and has no elixir (if you picked it up), make sure he is safely secure in effigy’s, singularity, frozen etc and hit him with both of your beacons simultaneously. This should destroy him and youll get your achievement.
The main part here is just paying alot of attention and if you followed the previous steps nothing should get in the way.