I use an open end wrench (I think 15mm?) with a breaker bar attached to it (Usually have to tape it to it so it doesn't slide off). And then position it in place and push forward (counterclockwise of course).
Have you tried spraying some PB Blaster in there? Sometimes it feels like you're not really moving it much but indeed you are. Maybe get someone to test the tension on the belt while you try? And have you done it before?
oh yeah, and
to Focus Fanatics!