I hit 43 Solo, I believe. Maybe 44. Co-Op (at the time of posting this) I have reached something like 19. Then we all died. I believe. My Co-Op scores are very iffy, because I don't keep track of those as much. A nice tactic I found for guarding perks solo is as follows: You buy Quick Revive to start out. Then, when you have enough points, you go and open the door next to Revive, and buy Juggernog, but do not open the door above Juggy. This way, you can guard Quick Revive, and Juggernog from the monkeys at the same time, since they won't come from above Juggy. Just stand outside the door, maybe place some claymores in there as a backup, and let none of them through.