The Mask cant be killed and also cant feel pain he has a shit loud of powers for all his crazy shit he could take on juggernaut and has taken on lobo he has stated that if he wasnt so lazy he would take over the universe but then they would be no one to play with.
Impossible Man can not only shapeshift, but also mimick powers whether it be special personal powers (i.e. strength, energy projectile) or mechanical (i.e. web shooter, gun firing).
The only time I have seen Impossible Man truly defeated (but it was not a serious fight) was at the hands of Warlock, who outdueled him by metamorphing into a character with color, something Impossible Man could not do (he's always green/indigo).
I don't think either one could defeat the other in a standard fight.
This was the great treasure hunt in one of the X-Men's annuals and I believe he also stole Selene's costume, but Galactus' ship doesn't sound right. He was not coaxing the X-Men for a fight and will not truly fight unless directly provoked.
Impossible Man doesn't take things seriously and more often than not, he's just a pain to be around and has annoyed the likes of Daredevil, Spiderman, Silver Surfer, etc.