Minion builds are really only unreasonable in 2v2s. 3v3 minion builds are a liability. Usually I've noticed the side with the pure minion build tends to lose (assuming equal demigod choices).
If you see a minion build, go heavy speed/armor/hp (ditch your mana, you really only need Scaled Helm or Nature's Reckoning against a minion build. Wand of Speed is more useful in these situations than Vlemish, for example). Spirits can't attack moving targets and you'll want to rotate flags as fast as you can. You'll also want to be able to chase him down because once he has all your towers down he'll try to push your portals constantly.
Journeyman's is very useful against minion builds since it will almost always be proc'd.
The other option is play Occulus, or TB. Both counter minion builds excellently. Occ doesn't get to really do it well until levels 8 or so, which is a bummer, but he's still a general.
Both Sedna and DA can counter Shield III - Teleport, though both of those are a poor 2v2 anti-minion build choice.