Yeah that TM clause is silly and forgive me for saying so, but kinda cowardly. It's like saying "I want to be able to completely predict anything that your Pokemon can do and make this as close to my in-game playthrough as possible." What is there to swing the momentum? You might as well just switch all day.
Anyhow, for actual sets to run, I'd put the following on Gren:
Gren @ Life Orb
Ability: Protean
Nature: Mild/Naive
Hydro Pump
Extrasensory
Low Kick
Gunk Shot
Protean will be absolutely crucial so you'll want to catch a Frogadier in Friend Safari and get goin. If you're not sweating EV's and IV's, I'd just take the first mild one with Protean. This was the best set I could come up with with tutor moves and Gren's standard learnset.
I'd swap Venoshock out for Shadow Ball and Psychic out for Substitute on Gengar, you can use Perish Song, Dazzling Gleam or Protect in the last slot depending on how conservatively you want to play.
You'll want some bulk on your team so Chesnaught is a good choice considering that he has decent synergy with Gren, but I'd swap out Mud Shot for Leech Seed and give him either Low Kick or Hammer Arm instead of one of your Grass-type STABs.
I'd recommend getting a Steel type on your team, probably Skarmory with Brave Bird considering the things you're already running.
And then a Rock type would also be good, if you're in love with Gigalith I'd recommend the following set:
Gigalith @ Salac Berry
Nature: Jolly
Ability: Sturdy
- Autotomize
- Rock Slide
- Earth Quake
- Protect
I know it seems stupid no TM moves, I've had battles where you use no items or no legendaries but to have a battle where you don't use TM I found it hard and puzzling because most of my pokemon have TM and don't forget in this battle I cant use items except mega stones and I think i've got a protean Frogadier somewhere