Missing achievement for Mission 11
I was playing this game co-op with my son. We went thru levels 1-10 no problem. At level 11 I received a phone call towards the end of the level so I dropped out. When I came back he had beat the level. I jumped back in thinking nothing of it, but after we beat mission 12, I checked my achievement list and realized I didn't get credit for beating mission 11. Is there any way to unlock the achievement now? Can I replay the level? Thanks for any help or advice.
Thanks to N3xus Gamer for the collector tag!