Battle subway help

hello.
i am quite the skilled trainer when it comes to double battles, but i cannot seem to even get up to 21st round in normal single train. i need suggestions/recommendations

here is my best team for single (got to round 20):

Wash rotom, Gentle
252spd/252spatk/4hp
Levitate
-hydro pump
-volt switch
-reflect
-willo-wisp

Regigigas (yes it can go in the battle subway), adamant
incorrectly trained EVs
slow start
-ice punch
-crush grip
-drain punch
-skill swap

Volcarona, Modest
incorrectly trained EVs
flame body
-psychic
-quiver dance
-flamethrower
-bug buzz

If I were you, I'd use this Wash Rotom for hitting hard or screwing over walls:
-Thunderbolt/Volt Switch
-Hydro Pump
-Hidden Power (Fire/Ice)/Will-o-Wisp
-Trick
Nature: Timid/Modest
EVs: 4 HP/252 SAtk/252 Spe
Item: Choice Scarf/Choice Specs

TBH, Regigigas is terrible, despite its good stats, due to its ability hindering it. Even though Archeops is hindered by its ability too, it packs all its power right off the bat, until it gets to 50% health or less:
-Stone Edge/Rock Slide
-Acrobatics
-Earthquake
-U-turn/Hidden Power (Ice)
Nature: Jolly/Naive
EVs: 4 HP/252 Atk/252 Spe or 252 Atk/4 SAtk/252 Spe
Item: Flying Gem

I'd re-EV train Volcarona if you already have EV reducing berries with you (you can get them from the Egg spin in the Wi-Fi room with 5 people). Volcarona is also best with Timid.

Thanks!
This has really helped a lot, i got rid of regigigas, and put my gallade in (which has a terrible nature and evs) and i finally unlocked super single!
however, a lot of the rest of what you said will take a while.

Here's the team I use for single train, its undefeated for me (haven't tried the super single yet)

I lead off with volcarona: hurricane, flamethrower, bug buzz, quiver dance. He takes out the first 7 by himself usually, sometimes more.

Electivire: thunderpunch, ice bunch, earthquake, brick break

Garchomp: earthquake, stone edge, outrage, dragon claw

Didn't list evs, items or natures. I don't worry about nature too much since it's not online, evs should be self explanatory, and everyone prefers different items