*Spoiler alert*
The last battle in the game is against the mage guy Zatalador (everyone in this game has stupid names) and the paladin Divine with a bunch of weaker enemies, however this is no easy feat. I was level 37 when I did this, and it was still a struggle in some ways even with my 1 -1.5k explosion arrows. You'll obviously want to deal with the weaker enemies first as the multitude of attacks at once can inflict a large chunk of your health. After this are finished I recommend taking out Divine, this may sound strange, however even with the wizard healing him, you'll find him easier than fighting the wizard whilst getting constant knockbacks from Divine. Slowly work through his health, keeping your own topped up and he will fall, then rinse and repeat for the wizard. After a few videos and some dialogue the achievement will unlock.