You need to beat all the bosses except for Darkrai.
There are NO Weepenbell bosses.
Bosses (I think) DROWZEE GROUDON LUXRAY MESPRIT GROVYLE DUSKINOR DIALGA THE GUILD FROSSLASS GYARADOS REGIGIGAS DARKRAI
no there is not a boss and here is something that might help you Pokemon explorers of time, darkness, and sky all do not have bosses.
None they don't work on bosses.
Ummm.... Attack them until they are defeated? Maybe bring a few necessary supplies during the fight like Oran Berries and Reviver Seeds?
There 11 bosses in total. (Including the White Wolfos from the Ice Cavern and Ganon and Ganondorf as separate bosses)
No, you can be one in explorers of the time, though.
Arceus is not in Explorers of Time or Explorers of Darkness. There is an Arceus statue in Explorers of Sky, but there is no real Arceus that will join your team.
their bosses over time
In Pokemon Mystery Dungeon Explorers of Time it is impossible to get Mewtwo. You are only able to get it in Explorers of Darkness.
Only in Explorers of Time you can be Eevee. I thought you (Hero or Partner) can be Eevee in Explorers of Sky.