For this achievement you'll have to beat Grunty at the end of the game. Not only do you have to defeat Grunty, you also have to destroy the drill tank that she's in. Grunty will let out small blue lasers from her drill tank. The tank will spin around, so you have to hop over these two lasers. After the lasers disappear, Grunty will pop out from the top of the tank and ask you a question. If you answer correctly, she'll use a slower attack on you. If you answer incorrectly, she'll use a faster attack. Enter the Breegull Blaster move and fire Grenade Eggs at Grunty at the top of the tank while avoiding her spells by sidestepping.
After Grunty has taken a bit of damage, she'll throw more lasers at you. Hop over these lasers and continue to answer her questions and shooting her down. After a while into the battle, Grunty will throw a mortar at you, dodge the missles. After this, just deal with some more questions and witch-shooting. Once again, Grunty will fire a second mortar. Dodge these and answer even more questions and continue to shoot Grunty.
Finally, something new happens, Grunty will start up the drill and it will begin to drive around in a circle. Avoid the drill coming towards you and the lasers being let out by Grunty. After that, Grunty's tank will stop. Take out your Clockwork Eggs and have the time bomb enter the exhaust port in the back of the tank. Inside the tank will be a bad guy and a battery. Just avoid the enemy and detonate next to the battery. Do this one more time and deal with a couple more questions and Grunty-killing. Grunty will throw some more guys out to attack you. Defeat the enemies and continue.
After Grunty's health hits 15, HAG 1 will start to let out toxic gas fumes. Before you die from the gas, quickly hit Grunty as fast as possible and hit her one more time at the end while avoiding her last few spells to defeat Grunty. After you beat Grunty (and the game, hopefully) you'll get this achievement.