This refers to 10 of the sub-missions; when you find a mission portal, there are always 3 sub-missions in there. So you only need to beat 10 out of 30 sub-missions throughout the whole game for this achievement. Mission portals are marked with a red star with an M in it on your map, and there are no hidden ones beyond what tools you need to get in there.
The earlier the level, the easier the missions, though you can't access the earliest ones until late in the game. If you want to minimize frustration, skip the missions in Casino (the motocross tricks).
Also worth saying is that after you beat the game, all shortcuts on your teleport map are unlocked, so you can just quickly teleport between mission portals to get this achievement done.