You're right Baggins, all golems are resistant to magic, though I think
how resistant they are varies.
I use arrows of biting to kill the golems, which works out rather well, because the sirenes near the flesh golem cave drop quite a few of them. So I slaughter the sirenes, take their arrows, and use them against the golems. They drop like flies
If you don't want to use these arrows, you can just use magical weapons - all you need is a +1 weapon or better to hit them. If you make sure your party gathers round the golem and all of them try and hit it, it will go down in about a round.