I just HAD to test this theory.
Grum is at his PEAK at L2- Void Armor+ Void means that, well, Magic can't hurt him. In addition, Fire has the Flame Monkey- which gets Repair at L5. Since there's no Tank Heal in Fire but there's no getting around Grum's armor- Repair is just as good.
Grum's weakness is his low speed, not just that he goes so late but that he misses a lot (A LOT) and his Bloodlust Ability only triggers when HE kills someone. To combat that, I paired Grum with a L2 Kitty (+2S, True Aim, Tank Heal) and in Gold- I started using a L6 Pyre/Supply Runner/Goblin Shaman/Ooze to speed him up, while slowing the other team down.
I wrote previously about Grum's adventures in Silver:
@axrho/testing-grum-in-silver
Where he didn't just do well- he SLAUGHTERED EVERYTHING! but once I advanced to Gold, I thought: "Can he still do well in Gold?"
I can definitively say: Grum is UNDER-rated. A L2 Grum rents for ~4 DEC/day on 
In short: Grum is a BALLER!
My first Battle with Grum in Gold was a Kitty/Grum vs Yodin/Grum SHOWDOWN!
https://splinterlands.com?p=battle&id=sl_3675c5e04dba38161f52bf49fd6e15a0&ref=rhosho1
The very next Battle featured Grum facing the 'unhittable'/'unkillable' Tigger: Kitty/Grum vs Chanseus/Kralus
https://splinterlands.com?p=battle&id=sl_49ed14d8079ab5db8e8c0cb3f06ca5e2&ref=rhosho1
Oh, Jesus..... Pyre/Grum vs Llama/Kron
Grum EATS Kron and doesn't even slow down....
https://splinterlands.com?p=battle&id=sl_b907eeb1f5d7931057733fda20348102&ref=rhosho1
My last Battle, in Gold with Grum featured a Magic-based opponent: Kitty/Grum vs. Magic (Delwyn)
https://splinterlands.com?p=battle&id=sl_0b3e74ace808746231aec1f8e4141b89&ref=rhosho1
Grum, through Silver and Gold was undefeated, unbeatable- one Battle, he never hit. Not once in the entire Battle. The opponent though, was Magic Based and couldn't hurt Grum. They spent the entire Battle trying (and failing) to bring down Grum while the rest of my team worked their way through the entirety of their team.
It didn't even matter what Summoner I used with him- Kitty, Tarsa, Pyre.... Grum, always won. ALWAYS.
