The achievement is unmissable and will unlock after fighting the boss in the burning desert region.
The fight itself is not that difficult, especially compared to some of the other boss fights in the game.
I used the following set-up:
Amon lvl. 7 with his blaster stance (bring enough water ammuntion)
Gnart lvl. 7 use his regen stance or bring enough healing Balm+.
Ive lvl. 7 use her electro lance ammo as much as possible.
How I managed to fight the boss effectively was to use Gnart simply as a health booster with healing balm+ potions, while Amon and Ive were firing away with water and electro ammo.
It definitely helps if you used the ammo master perk on Amon's and Ive's talent tree, but it is not necessary. The fight is easy enough
If you prefer a video guide, you can use the one below. A different set-up is used in the video but I think it might be helpful as an additional orientation.
Jump to 33:11 to see the fight. (Video credit: Plagueis Xboxlivefr)
Hope this helps you guys!
Any improvements? Please let me know in the comments below!
