Roboquest

This achievement is only for the Guardian (yellow starting robot). He can get a perk that'll change his RS click "Bash" to a giant hammer "Bonk Hammer". Once you get this perk, you just have to get lucky with your boss RNG that'll let you get the boss called Diggy Mole. He's mostly showed up for me in act 1 boss (Canyons/Oasis) but I've also had him show up in a later area I think.
You can whittle him down with your guns, but you just have to make sure the last hit is with the hammer. It does considerable damage even without upgrades to it.

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To obtain this achievement you need to use the Guardian class, as one of the perks you have a chance of choosing when you level up is the 'Bonk Hammer'. This replaces your normal melee ability with a hammer and to obtain the achievement you need to kill the boss with the hammer. You can use guns to bring the bosses health down as long as the final blow is the hammer. I'd also make sure you don't pick up any items which do damage and could possibly kill the boss before you finish them with the hammer.

It is random which perks will appear when you level up but out of the 4 attempts it took me I got the Bonk Hammer on 3 of them. Keep a look out for the 'Dice' item being sold or appearing in boxes as this allows you to re-roll your perks twice if you don't get the hammer.

I'm not sure if the bosses rotations are different for everyone but I had the following order - Diggy Mole, Dr Turret, Billy Boom and then Diggy Mole again. I ran the same route of Canyons to Oasis every time for 4 consecutive attemps and this was the order of the bosses and one the 4th attempt I had the correct boss and the hammer.

I never tested this but you may be able to load in to a new run and simply exit straight away if you know the order of your bosses and then do this until you get the correct boss and then attempt to get the hammer.