i did some 7-4 runs recently but im not really good on instakilling benjamin, how would one achieve this

first point: don't refer to it as that /lh /hj

jokes aside, assuming you're referring to lobotomy, that is done on patch 14a (Violence release patch). there's also a way to do it on 15d (last Full Arsenal patch), but there is currently no way to instakill the Earthmover on any post-revamp patch.

lobotomy works by entering the brain room from the outside in such a way where it activated the laser defence grid. because the Earthmover is still moving around on the outside, the lasers will begin to damage the brain, eventually killing it. hence the name "Earthmover lobotomy".

if you're just referring to standard brain kills, i'm not too sure personally. I'd ask in the speedrunning Discord if you can, and if not, watch a few runs at a slower speed until you're able to understand how the kill works.

if you are referring to the kill used in WR then here's the breakdown: shoot saws up into brain room before reaching it, then shoot an oversaw and attach jumpstart when the brain spawns. fastcoin 2 coins while walking backwards, punch coin -> switch fist (cd reset) -> punch coin. throw a 3rd coin into the brain and shoot it with the Erail, throw 4th coin while walking backwards and punch it as the Erail targets it (RCP). slarp and cannonboost if needed to finish it off