AFAIK, you can't take Manticores or Deathstrikes in an Armoured Battlegroup precisely for this reason.
If you want evil, try the Destroyer Tank Hunter with slick loader with 2 LRBT with slick loader.
Actually this is not an armoured battle group!
Grizmund makes the crew skills available for tanks in any force led by him
You evil, evil man...
Firstly, I'd throw this one by the TO and see what he says. Technically Grizmund allows tanks under his command to take multiple
crew skills, it never says he makes them available
for his tanks. The Ace Crew skills are under Armored Battlegroup Special Rules, implying that these rules are only available for tanks in that army list. In this scenario, Grizmund is just a Leman Russ that counts as an HQ choice.
You could say the Grizmund drags the rules with him since he allows his tanks to take multiple skills, but nowhere do his rules say this is the case. If the TO says you can, then you might as well give your tanks ace gunner as well, because people will be screaming cheese anyway.
As for the choice of LRBT vs manticore, I'd go LRBT. They'll still make your opponent eat his codex as youre firing twice as many battlecannon rounds as usual, but the Manticore tends to kill whatever it shoots at anyway. Besides, you could always do both. With Grizmund that means you have three LRBT and two manticores firing twice... Again, if you're using slick loader, you might as well go all the way...