My general tactic against Barons, if the floor has gone on long enough, is to assume they have some kind of item on them. Med-packs are going to mean that you don't want it reaching "(almost dead)" or around there, which is when they use larges (smalls aren't worth worrying about). Armor doesn't matter TOO much, especially because it often slows them down enough to offset the DR (furthermore, it's easy to tell when they do and can switch weapons for it). Phases and homing phases...yeah, that'll suck, but those items are luxury more often than not as it is.
When it comes to weapons, there are plenty of possibilities. Rapid-fire is plasma, which is obvious. Shottymen have the double, which is almost always enough to close the large med-pack gap (and half of the time it's an MAD build...MFa should understand well enough that running is oft-used) provided you time the attack right. Pistols are only a concern against armor-whores, as they have the firing rate to compensate for low single-volley damage. For any game, there's the rocket launcher for high burst.
Games that will see the most impact are AoMr/AoSg (depending on build) and AoPu. In a standard game, I stand by my previous assertion: it's only in the absolute worst of cases where you find yourself forced to kill a nigh-invincible item Baron, and not nearly enough to dread.