Honestly? Some of the EASIEST achievements are the most rare, and I don't know why... The multiplayer battles you can play vs. the AI. Like the siege battles, just put a unit of native american archers or axemen in a wooden fort, then attack with however many troops you want. Then do the same for defensive sieges (one unit attacking say a star fort). Ships? Easy. Just put the lowest possible ship against a pack of sloops (they're fast).