Lets start off with our worst-case scenario. If we can find a win there, we can find a win anywhere.
What is our Worst-case scenario?
Barbarian wins initiative
Barbarian gets into melee range and attacks.
Barbarian hits, crits, confirms, and rolls max damage.
Assuming 22 STR (18 base +4 rage) and full power attack, we're looking at:
Greataxe +7 (+1 Enhancement, +6 Strength, +5 BAB, -5 Power Attack) melee, 1d12 + 20 (+6 Str, +3 two-handed (1/2 STR), +1 enhancement, +10 Power Attack)
Max damage is thus 32, which is 96 on a x3 Crit.
So, the Barbarian is capable of slicing the wizard in half on round one if he gets lucky.
How do we prevent this from happening?
Edit: Also, what kind of prep time do we have? Do we start the battle with buffs or no? Hour/lvl+ buffs only?