variation 1)
partykill lock (locks current kill, no other kills will overwrite)
partykill clear (clears current partykill)

variation 2)
There are a couple of biggest kills in a list you can choose from,
a bit like with rank (rank list, rank 4). Maybe you could drop some
of these too.

Rationale:

The exp worth of the mobs doesn't really describe their toughness,
minus few exceptions. Also, many many places have bigger "blockers"
than the actual bosses: Kitiara is bigger than Soth, Eater of the Dead
is bigger than Moreod, Rotahunt is bigger than Tyranicus, etc.

I just killed Moreod, I want Moreod as my partykill, not the sucky
Eater of the Dead.

-Era of the tinmen race