Testament said:Animal Growth is irrelevant on the Druid, if they're going Brown Bear, it doesn't stack. The spell even says "multiple magical effects that increases size do not stack". AFAIC, wild shaping/polymorphing yourself from a medium to large creature fulfills that critereon.
Anyways, attack them with a group and grappling becomes somewhat more dangerous. Especially if there's a rogue who can sneak attack the grappling bear. Flyers are also a good choice. And freedom of movement on a melee type to just say "no" to the Bear.
IMHO...
Wild Shape isn't a size alteration, its a form alteration that may coincidentally alter size. When you wild shape, you ARE the base creature minus some special abilities. You then can increase it once from the base creature.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.