1) First you buy some "Noggenfrogger elixers" from Gadgetzen.
(Makes you skelet/small/Slow fall)
2) Then you drink some of them till you are small, tiny, whatever you call it.
Its random, one time you get skelet, other time tiny. Keep drinking!
3) You go to the arena in STV, stranglethorn vale. Better to be lvl +40!
4) Drink a pot 5 minutes before the chest spawns
Chest spawns at 3am, 6am, 9am, 12am, 3pm, 6pm, 9pm, 12pm
5) Go to the spot WHERE THE CHEST SPAWNS!
Its in the middle of the area, can't miss it
Another good trick is to do step 1-4 above and log out on the spot where it starts & log back in when chest spawns. Let everyone get killed & open chest while they fight.
Enjoy!