The hammer should not have any slack or slop in its fit with the tumbler.
It sounds like the square drive in the hammer has been damaged so that it no longer fits the drive square tightly like it should.
The tumbler and its drive square are hardened (or should be) so there is nothing that can be done to that item.
That leaves the inside square in the hammer as the only thing that can be modified to improve the situation and IMO most of the fixes short of replacing the hammer are somewhat marginal.
Things that have been tried are tinning the flats in the hammers square with solder to reduce its size. The solder usually brinells or becomes displaced after a small number of shots because of its soft nature making this a temporary fix.
Using a fine pointed punch with a small hammer to form a multitude of indentations into the 4 flat surfaces in the hammer often works better. At each punch mark the material that was in the mark becomes dislodged and flows upward effectively reducing the average size of the square hole.
This is a "more is better" thing as the more places where the displaced material exists, the more actual contact will be present between the hammer and the tumbler.
Another thing that can be tried is to buy a .001 thick steel "feeler guage".
Cutting this to fit into the square between the hammer and the tumbler will be necessary.
Then use this on two adjacent sides of the square while driving the hammer into position.
Often, even though the .001 (one thousandths of an inch) is very small it will be too thick to allow the hammer to be assembled.
The best idea is to try to get a replacement hammer. At one time, Deer Creek had some parts for these guns so perhaps calling them would be a good idea?
Deer Creek Products
6989 E Michigan Rd
Waldron, IN 46182
(765) 525-6181
