I guess it's my turn to shine, as per said by @StaticShock, me and/or @KingOfAllTrades may help you out, so you have 2 guides to follow here:
Note: You must have some nullifiers and must have 4-5 gp 3 may be possible but i havent tested it yet. Also with plat cards
First of all, why would you play Grimm on hard mode, that may seem impossible, but it is possible with a little bit of RNG, first of all I will list his team with some items and abilities
Note: This is not in order so this may vary.
Grimm's Smeargle: 5 GP with a platinum card
Ability: Wonder Guard
Item: Gaia Nullifier
Strategy: Well, this is impossible to get by with using normal means, so you must either have some pokemon with some ability to trap the opponent so that they can't switch, such as Shadow tag, either that or you may use Matrix Compression. The only way is to let it Struggle to death or PP stall to be exact. Or make it blind or use an effect on it.
Shiny Darkrai: 5GP with a platinum card
Ability: Bad Dreams
Item: ?
Strategy: Very easy to handle tbh, just use some effect moves such as poison, blind, or burn, or use other strategies you may have.
Shiny Ho-Oh: 5GP with a platinum card
Ability: Pressure
Item: ?
Strategy: Same as darkrai, use other strategies if necessary.
G-Ducklett: 5GP with a platinum card
Ability: Godlike
Item: Nothing
Strategy: Oh boy nothing can tell me how hard this is, same strategy as for smeargle, BUT ONLY USE special moves, it is a must to lower its special defense, other than that you're good
Shuckle: 5GP with a platinum card
Ability: Unknown
Item: Mega stone for Shuckle
Strategy: PP stall it, this abomination has a defense of half a W.T's, not even exaggerating. Or use some poisoning effect on it, your good for now. Or a sleeping effect.
Last Pokemon, the hardest one of them all, if I haven't said this, do it now, SAVE BEFORE FIGHTING GRIMM.
??QMARKS??: Platinum form with 5GP
Type: Ghost
Ability:Godlike
Item: Nothing
Moves:
Delete Matrix
Matrix Compression
Hard Crash Drive
Freeze Bug
Strategy: Have a pokemon with substitute and a ghost type move which isn't a physical move Preferably OM_Crobat with a Frost nullifier, use substitute, hope to the rng gods that it does not use hard crash drive , hope that it only uses matrix compression and Delete Matrix, if you kill or kills itself, good job, you finally did it you beat the 3rd hardest trainer in the game
If you are wondering why I only listed Qmarks moves and type, it's a "Special" pokemon
And if you are also wondering, Delete Matrix has 2 forms,
1 is that it has 10 PP, and deletes the pokemon from existance on your team, making it useless forever, making it level 1 and no moves, and Deletes your whole entire bar in a nanosecond
2 is that it only has 1PP, and does the same thing, but it still uses the animation for the damaging part.
And also your welcome.
Edit: Some pokemon you might wanna use are
Porygon-Z: 4GP with shiny card
OM_Crobat 3-4GP with shiny card
and
Wiseraph 4-5GP with platinum card