You have a lvl. 73 Chimchar? Many strategy people post here are useful, I just hope you can use mine.
Your team is like mine, I'm a Torchic (Lvl.53 at that time), and my partner is a Chikorita(Lvl. 52 at that time), oh and Cresselia.
The first thing you do is protect your Cresselia, She's so useless! D:
Also, watch out for Magmortar, His Flametrower can kill you at one shot.
Anyways, trutwig is just fine because it has Razorleaf right?
Rock moves are good, but you won't need it much, just use a rock move on Pokemon who has Flame Body and Flash Fire. (My Torchic has Rock Slide)
For Boss battle, take care of Arbok, Mismagius and Magmortar FIRST. they're a real killer.
Then make Cresselia take care of Aggron, it will keep her company (Cresselia won't die, Aggron has lame moves.) And make turtwigtake care of Rhyperior FIRST, before the rest.
For Darkrai, Does Chimchar has fighting moves? well use it, My Torchic can't use fighting moves after he evolved. x(
Flametrower and Ember is a real life saver in the dungeons, Seeds and Flamethrower for battling Darkrai.
I DO hope it helps, your team is 20 levels higher than mine, and I defeatedit the first time. x(