KABOOOM! achievement: how to get it in Die of Death

"I have one last card up my sleeve!" See the requirement, reward, evidence status and practical checks.

Direct answer

"I have one last card up my sleeve!"

Category: secret

Library category: secret.
Recorded requirement: In a public server as Killdroid, trigger the low-health self-detonation while using Flight and kill another player with the explosion. The source says it does not work in Dienation and warns that the timing is extremely difficult.
Recorded reward: 400 Points & Wardroid Killdroid Skin.
Unlock quote: "Shocking."

In a public server as Killdroid, trigger the low-health self-detonation while using Flight and kill another player with the explosion. The source says it does not work in Dienation and warns that the timing is extremely difficult.
Check Library > Achievements after the round; a Roblox profile badge is not required for every in-game achievement.
If it does not unlock, recheck the exact role, mode, player-count and public/private-server conditions before retrying.

This requirement is documented by the community Achievements table, revision 87931. It is not a developer API and may lag behind a patch.