Beorn takes a reflexive swing as the beast gets within reach, then taunts it as he readies another blow."Back off cow."
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AOO (can make flat-footed due to muscular reflexes)
(1d20+11)[23]
damage (1d10+8)[10]

challenge (swift via swift guardian) bull takes -3 on attacks against anyone else, gets +2 verses Beorn for 5 rnds


standard to patrol w/ punishing rebuke (attacks against allies provoke)

iron wall gives all the bull 20% miss chance against allies, 50% if they are within Beorn's reach (20 ft). Beorn doesn't get this miss chance.

Berserker for -2 AC, +8 THP

in short, attacks against allies takes -3 and suffer 50% miss chance and provoke an AOO from Beorn
Attacks against Beorn get effectively +4 (AC penalty and attack bonus combined)