WOAH, STOP!!!!!
A mountain bike cassette is completely different to a BMX freewheel. You just have to undo the cassette lockring away from the freehub body using a chainwhip to hold the CASSETTE and a lockring tool to undo THE LOCKRING.
My advice? take it to a bikeshop, they should have ot off in 5 mins and a decent chainwhip and lockring tool are a big cost for a one off job. Otherwise, clamp the non drive side axle into a vice (preferably using specific axle grips) just to give you a nice working platform. Then hold the cassette in one of the middle sporockets and simply hold in place while you undo the lockring. DO NOT CLAMP THE HUB BODY, YOU'LL PROBABLY SQUASH IT!
With a BMX hub, do as others have suggested, rebuild the non drive side to give you the leverage from the rim. HOWEVER as also rightly pointed out, the bullseye hubs are made of multiple pieces so you need to try and rebuild on the drive side if you can, if you can't get spokes through (although with a bullseye flange and a 16T freewheel, you should be able to rebuild the drive side. If not, as others suggested, destroy freewheel in vice and remove that way.
