The description of this one is a little confusing. You need to get 50 wonder victories, not just build 50 wonders. Also, a wonder victory against 7 other players counts as 7/50 instead of just 1/50.
You can easily get this by playing any game mode with standard victory conditions against 7 of the easiest AI. Post-imperial death match or maybe turbo random map as Spanish is probably fastest. Have 1 villager build ~10 houses, 2 build a couple extra TCs, and the rest work on the wonder. You can fit roughly 40 villagers around it as long as it's not up against anything. Once your 3 villagers are done with the houses and TCs you can have them build a few castles just to be safe. Save the game with 1 year left on the victory counter, let it run out, and then just reload as many times as you need, should be no more than 8.