Guaranteed faint? You've forgotten about the magic of Trick Room...
Treewig, since we have Trick Room, you can get off a STAB Overgrow Mega Drain x2 combo on Riolu!
Ralts, since they want you dead right after Turtwig, use Psychic to grab Vulpix and throw it at Charmander. Then, use Heal Pulse on Torchic to keep it alive a little longer.
Torchic, use your signature combo on Vulpix! (Night Slash + Reversal)
Turtwig: Mega Drainx2 Combo (Riolu)->cooldown
Ralts: Throw Vulpix at Charmander using Psychic->Heal Pulse (Torchic)
Torchic: Reversal+Night Slash Combo (Vulpix)->cooldown