Both FAQs list Riolu as learning Force Palm at 15 and the one that talks about level 11 mentions Counter being learned at that level.
Anyway, as others have said, Counter is really all you need. One Work Up + Tackle from either of Cheren's Pokemon will be the downfall of themselves because Riolu can at least take one Tackle and Counter it for massive damage. Personally, I used the swap to heal Riolu and have my other Pokemon Tail Whip and hit the Lillipup down enough that Riolu could one hit with Quick Attack but not enough for Cheren to use his Potion.
Why would you use the FAQs on this site? Text documents are user unfriendly. Just go to Serebii.
I... don't? I just checked them to see what the TC was talking about. I've already gone through White 2 Japanese version blind. :s