From my experience:
Counters to Wolverine:
1. Air
2. Counterartillery (Firewalker, Tremor or Catapult)
3. Napalm missiles from Silo
4. Raiders (won't work vs decent Wolverine play, but can work when enemy screws up or is incompetent).
5. Assaults (very situational, may get skirmed to death, really depends on the specific unit and the wolverine count)
In my opinion the best, most sensible way of dealing with small-moderate amounts of Wolverines is air play. Any kind of bombers can work, choose the one that suits the situation best (Do those Wolverines have some AA support? Does the guy tend to clump his Woverines? Do you have lots of fast units to follow a Thunderbird strike up with?).
If there is significant AA support with Wolverines or their numbers have grown very large you obviously need something else. This is where certain rarely used artillery units can save the day. I've seen Firewalker, Tremor and Catapult all be very effective vs heavy concentrations of Wolverines. They are all expensive and fragile units so handle them with care. You should know what you are doing or you're just throwing metal away.
If you think your arty is going to get sniped from the front lines you can also build a Missile Silo. It has great range and can build Inferno missiles. A few good hits with those can decimate Wolverine armies.
Raiders are not a reliable counter to Wolverines due to the mines and the likelyhood of a good vehicle player always guarding his Wolverines with other units or static defenses. Don't go for this unless you are desperate and don't have the metal to pull off the more expensive and more effective counters.
Assaults can, to an extent, shrug off Wolverine fire and be of some value when enemy has Woverines. However, they are not a good counter. Wolverines are faster than all assaults other than the Ravager and are due to that very potent skirmishers. They will eventually kill your assaults unless you can constantly repair them. Even if you can repair them, Wolverines can then just kill your cons first and then continue to kill the rest of the army.
Countering Wolverines is a pain, but not impossible. A common mistake people make is spamming regular units vs Wolverines. It rarely works. You have to get your counters out or you will die a slow death.
Edit: as mentioned above, Pillager is ok as well vs Wolverines. However from my experience Tremor is better. Dealing with very heavy Wolverine spam is one of the rare cases when Tremor can be a good investment.
