I guess there is no TH combination / Sound in Polish...
An easy trick that I devised to work with Polish speakers trying to learn how to pronounce "th" is to make them aware that this is the exact sound that is produced by Poles with a slight lisp whereby they cannot pronounce "s" or "z".
For instance: "sasanka" (pasque-flower) , or "zaraza(pestilence)" pronounced with a lisp as voiceless "thathanka" or voiced "tharatha respectively"
Nearly all Poles I know can imitate the lisp, and once they are made aware of the similarity, they are on their merry way to use "th" it should be used.