A Tree-Killer party starts with a blocker, an exile with enough defense to ignore or take six Tree Giant blows in quick succession. The other hunting party members areattackers and stand back, while the blocker taunts and teases the Tree Giant into lowering its balance.
(How? Figure that out for yourself; if you're experienced enough to hunt Tree Giants, it should be obvious.)
When the Tree Giant has lowered its balance, the other fighters (and the blocker) all swoop in and swing madly. It takes 15-20 axe blows to chop down a Tree Giant. Assuming a hunting party of 5 mid-level fighters, the rain of blows from all directions should topple the monster.
Now, if all the fighters are cross-sharing with each other, how much experience is dealt out?
For a grand total of 70% for each fighter, or 350% of what a solo killer would earn.