If you, the OP, has a Harbor Freight tool store close to you, you can pick up a watch press that should work. It's one of those odd items that HF will stock for watch-work. I have run across a few of those cases that are an absolute bear to get the back on without a press. The press isn't expensive, and has plastic dies that won't mar the case. You just need to pick the right size that contacts the bezel without pressing on the crystal and a press side that makes contact near the outside edge of the back.
Other than that you'll probably have to improvise with a pair of channel-locks or a vise where you can get leverage. The trick will be in figuring what to use that protects the watch as you apply pressure. As I said, the HF tool isn't expensive. Good luck.
