Team Deathmatch

These are the champions (NPC) who come out from the other Faction ship against you, they spawn randomly in Normal, Heroic, or Mythic, so just keep farming Heroic as I did and you will get them all.

Achievement is bugged for me. I killed Razak Ironsides twice but didn't get his name crossed off when i did.

Listen for a voice making some kind of threat aimed at the Horde (I hear it about halfway through the expedition). Once you hear it/see it in chat scroll, look for three blue Alliance badges moving around on the map (they tend to be together-ish), to help locate them. As another poster said, they can spawn at any level (normal, heroic, mythic). You don't need to be in pvp mode and they appear in solo expeditions, too.

The Horde version comments for this achieve list the three champion NPCs grouped together. That's helpful if you just need one or two more to get the achieve. Here are the Alliance champ trios (found in https://www.wowhead.com/guides/island-expeditions-advanced-ai-teams, which has more details):

- Razak Ironsides, Dizzy Dina and Tally Zapnabber (I found them in Rotting Mire).
- Shadeweaver Zara, Riftblade Kelain, Duskrunner Lorinas (found in Molten Crag)
- Varigg, Stabby Lottie, Briona the Bloodthirsty
- Squallshaper Bryson, Squallshaper Auren, Frostfencer Seraphi
- Raul the Tenacious, Gunnolf the Ferocious, Fenrae the Cunning
- Vindicator Baatul, Archmage Tamuura, Anchorite Lanna (found in Skittering Hollow)
- Airyn Swiftfeet, Lady Tamakeen, Brother Bruen
- Thornstalker Nydora, Thornguard Burton, Thornspeaker Tavery