NieR

This boss is indeed named Wendy, not that anything indicates that in-game. You will know it as a gigantic floating eye with magic blocking shields rotating around it in a semi-random pattern. You have to hit it right in the eye with magic at most points.

Don't try for this on your first playthrough. It is a long fight and your magic will really not be strong enough to get this done fast enough. You'd also certainly have to skip the cutscenes which are pretty interesting plot.

(EDIT: using Magic Drops consumable to boost your magic damage by a lot makes this viable on your first playthrough - thanks to m0rph3us17 for the suggestion)

On your second playthrough- go for it. Skip all the cutscenes with start during the battle as usual when going for a time trial kill. I liked to charge up one or two dark lances (it goes without saying to have words that increase magic dmg on all your spells, and for dark lance, since it is high cost, you want to make the secondary word attached to it a magic cost reduction - hopefully you picked up the 30% reduction word by now) and when the eye is uncovered for a second AND I could tell a shield wasnt going to pass over it in the next couple seconds and then let them fly. If you charge up more lances than that you might get stuck waiting for a shield to pass over the eye and with the slow motion effect of charging magic you will end up charging your entire bar waiting for the shields to pass most likely. That's why you only go for one or two at a time.

You will shoot it for a bit and it will come close to the platform- if it's not already fully damaged for this phase you MAY be able to hit it with a melee weapon in the eye, but at this point it was already fully damaged for me and no more damage could be done until the next phase. Emil will shout "Something's coming!" and the eye monster will flip around and wrap up its shell and start charging its laser (IM'AH CHARGIN MAH LAZOR). You can start running toward the top bridge as soon as Emil says "something's coming!" - you DONT have to wait on the platform for him to shoot his laser and intercept the eye's beams. as soon as you get to the top bridge it will be handy to have a fast 1h sword equipped like the ancient overlord or something to cut through the periodic energy ball waves it will fire backwards at your position. Again, here, just shoot dark lances right into the eye. It will quickly wince and fly downwards as the dark lances cause massive damage to it.

As soon as it starts flying down or stops taking damage from your shooting it in its open eye, that is the end of that phase and you can start running towards the platform in the middle of the first bridge in the Aerie. From here, maintain a high level of magic charge and shoot probably single lances or maybe even use dark blast until the weakened countdown clock symbol appears over the central eye. That is when you want to charge up as many dark lances as you can so you make sure you shatter the clock the first time it appears and that will end the battle and go into several cutscenes which I recommend skipping all of them. You've seen them before if this is your second playthrough. I believe the achievement pops when you regain control of your character and are standing at the edge of the cliff by the mailbox.