It's not overly difficult, but you do need a special socket to get the yoke fixing nut off.
Several threads with the actual tool and hand made versions around (differential seal would likely be a good search keyword candidate)
Are you positive the 13mm is what fits? If it's not perfect, you'll end up hating life when the plug strips out where the tool goes.
If tapping/rapping at it with a hammer doesn't work, you might try a short cheater pipe on the wrench, or, heat up the area around the plug with a propane torch then touch an ice cube onto the plug itself to use the heat expand/cold contract effects to your benifit.
They can be pretty darn tight, but once you get them moving they tend to come right out.