the freehub is aluminum? Does it appear to have deformed splines from torquing the cassette onto the splines?


Anyway, use some lube on the splines, and you can install the wheel on the bike, rest the bike on the chain side (careful not to rest on the derailleur), and from between the spokes tap with a wooden piece into the rest of the cassette (cogs), you can install by hand the lockring so when it fall through it wont reach the frame. If tapping in several spots does not work than try taping at two opposed spots at a time (Two wooden dowels from the non-driveside) with a block of wood resting on both dowels and punch it with a hammer. It will come loose.
Or if you have use a 3jaw extractor, like a bearing extractor.

If the spline appear to have some grooves from torquing the cogs onto it, you can first put back a few cogs and the lockring (handtight) and with a chainwhip hold of an independent cog an twist a bit the block of cogs (with another chainwhip or taping with some wood tangent on one tooth) in trigonomoetryc direction - that should pull back the cogs from the grooves of the splines.