Basically, when Fippy dies in Qeynos he goes to Brell's Rest which is like where all of the gnolls and kobolds and dwarves etc go. So he's more powerful there for on reason or another. What happens in the fight is he continually "respawns" back in Qeynos thus despawning in the raid. So while he's hypothetically doing his thing in Qeynos, you get to deal with some undead adds. Then when the guards in Qeynos kill him (and he zone of course) he comes back for more.
Something the devs wanted to do with this was have a mechanic in the raid that turned you into Fippy in Qeynos and the only way to get back to the raid was, well, you know
(as the devs put it)