What I did was, run around the edge of the arena. The fire pig would not be able to charge at me because I was constantly moving and there was no space. At some points the fire pig would sneak a charge attack at me so I would dodge it or jump of the arena. As for the smaller pigs, I would hit them with a charged ice attack which kills them in 1-hit. The spear enemies were the least of my worries as I was moving to quickly for them to hit me. It took me 3 deaths to kill the fire pig. All other bosses were cake.
If it helps I also did this in co op. I had my brother constantly shooting ice arrows at the enemies causing a distraction. Using co op for the last level bosses really help out. Doing it in co op does not void any achievements.
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