The old sniper-duel achievement was triggered when killing a skeleton at a minimum distance of 50 blocks. The new sniper-duel advancement instead can be triggered by killing a skeleton at close range and subsequently killing any mob, skeleton or not, at a minimum distance of 50.
- Revoke the "Sniper Duel" advancement in case you already have it
- Give yourself a bow and arrows
- Shoot & kill any mob (except skeleton) at a minimum distance of 50 blocks
→ You are given the "Sniper Duel" advancement
Include "type": "skeleton" in the "entity" section of sniper_duel.json.