First thing comes first if your level of pikachu is low get its speed higher by leveling it up once it gets high use a thunder stone.
-For Blazekin definetly add hi jump kick instead of flamethrower since you only need 1 fire type move
-Id say your torterra is fine.
-Add agility on pikachu and take away anything you dont need.
-Your greninja has great moves.
-For garchomp there isnt a whole lot to do since his best things are dragon but if you really want to change one of its moves get rid of iron head.
-For crobat I know X scissor has good power but its bug it wouldn't come in very handy if you get stuck in a situation.
Now for the Natures:
You probably won't like this but the only Pokemon with a good nature is Crobat
Since you have one chance at this or you lose your Pokemon your best option is to use something like the gts but I would keep your blazekin Greninja is also good keeping since their hard to get same with torterra. Get pikachu a static ability and an Adamant Nature with a light ball. I'd keep Garchomp if I were you same with Crobat.
For the ev training if your going to do it.
Blazekin: 252 Atk, 252 Spd, 4 Spd.
Greninja: 252 Spd, 252 Atk, 4 Def Only do attack and def if you want.
Torterra: 252 Atk, 252. Def, 4 anything else.
Pikachu: 252 Spd, Anything else on others.
Garchomp: Since hes jolly give him speed 252 Spd, 252 Atk, 4 Hp.
Crobat: 252 Def, 252 Spd, 4 Hp.
Unfortunately Most of your Pokemon are Speed but I guess thats good if you dont want to lose If you can get an Adamant Garchomp.