Two things to try- first, a rubber jar lid gripper to try and turn the front retaining ring. Failing that, you need to get a variable diameter spanner wrench. Look for them at your local hobby shop to find one that is small enough and precise enough, or failing that, try eBay.
I will look for a rubber gripper something like that to try. Hopefully it will not scratch the lens barrel. What is a variable diameter spanner wrench? Will it likely to leave a scratch on the lens barrel?