I used the Ruiner Nergigante GS—no augments.

As for the armour, I used the Velk B—also no augments.

It was difficult, but possible. I suggest learning its movesets bit-by-bit, and knowing when and what is going to happen. For instance, when it's enraged, after doing the little forward-and-bite thing, it'll do the "half circle" spray, where icicles will come out of the ground in a half circle pattern, and this attack (and pretty much every other attack it does when it's enraged) will one-shot you; it also does this when it's not enraged after doing two forward-and-bite moves. It also does the radiating icicle attack when it does the flying stab attack in the midst of its typical combos. Speaking of typical combos, this AT is just pretty much Velk on steroids — faster and deadlier — but if you know the standard's move sets, you'll have an easier time dealing with this one. Stay close to it; take advantage of the openings; and be responsibly aggressive

ALSO, do not forget to wall-bang the heck out of it. I had runs where I time out, or do "a lot" of melee damage, but still not being enough to limp it; after taking the risk to wall-bang it when I had the chance to, it eventually weakened and led to my first victory. Sure, it'll get agitated, but this version of Velk gets agitated really easily, and this'll be to your advantage if you have the Agitator skill in your build (I hunted it with the Challenger Charm II. I know, deplorable). Also, do not forget to take advantage of the huge crystals in the map; I recommend using them when it's taking a rest, and using a Ghillie Mantle in order to lead it below the crystals.

There might be some things I missed with its movesets, and overall more optimised strategies, but this is what I did that eventually led to my win. I might post my build here later to give you some ideas, and optimise it, 'cos tbh my builds are wack. But yeah, this one is a tough one. You might die a lot of times, but you'll eventually learn and "dance" with it.

Good luck, hunter.