Picked up a Crossman Phantom last week at local store. Needed to kill varmnints in parents garden. Gun came with a scope.

How accurate can I expect this gun to be at 25 yards ?

So far it has not impressed me.

I am used to shooting modern firearms and am new to air rifles

You will have to try different pellets to find out which ones your air rifle likes. If they are to loose they will not be accurate I have found with my Beeman R9 this to be a problem. I also prefer to shoot a heavier pellet it may drop faster but it hits harder (retains more energy) than a light pellet.