The best AA in RA2 is the Aegis Cruiser, it brings Kirovs down pretty fast, even alone. However Naval units and water must be present on the map for that.
Grizzlies can't take that much damage to stay stationary for infantry covering. They will be using their speed and manuverability to win a combat, so basicly even if infantry could hide there, there are two possibilities:
-The driver is an idiot, trying to absorb all the damage it gets with its low health to defend an infantry unit.
-You are the idiot, hiding behind the Grizzly, then the driver simple moves away, because he knows he'll die fucking quickly when stationary, then you are left without cover.
And as rm pointed it out, worldbox doesn't let you hide there anyway.