Heat. I used a gas oven set to 200,turned off after 5 minutes. Let it sit for fifteen minutes.

Then use a hose clamp. wrap the lens barrel in rubber, like used for plumbing gaskets to protect it and give the hose clamp a grip. Don't tighten the clamp too hard, enough to get a hold but nothing more. If you need to separate the two front elements, use a second clamp. If you position the two tightening sections properly, you can then squeeze them with pliers to pop them apart.

EDIT: I now note that you DO need to separate the two front elements. Two hose clamps. Off set the bumps and try to pull them together (first element bump offset clockwise looking from the front). Don't forget the rubber!!