Not quite invincible. A Pokemon with Wonder Guard can still be hit by status. The most obvious method would be to use Toxic. However, things such as Sand, Hail, burn, and such hits just fine. Other forms of indirect damage such as a Rugged Helmet will also damage.
Then you have Mold Breaker and its variants.
You can also remove Wonder Guard through a move such as Gastro Acid.
Struggle can also damage a Pokemon with Wonder Guard, although this is not very reliable seeing as more likely you'll end up KOing yourself.
And Fire Fang also for some random reason hits a foe with Wonder Guard and damages them, although only for Gen IV.
Beat Up can also hit in Gen IV.
And yes, as Ninja said, Wonder Guard you can't skill swap, so Trace would have to be the method of obtaining it.