These words won't be appropriate.
Hit me _______.
What should I write as opposite of softly here?
Hit me hard
seems to be the best option. It's opposite to 'softly' or 'gentle tap'.
Jasper gives us a very good point. 'hard' is different from 'hardly' so one should not get confused that opposite to 'softly' is 'hardly'. 'Hardly' means barely.
While "hit me hard" is perfectly acceptable, if you wish to emphasize that this is a challenge, you might try "Hit me with your best shot."
You could also try "forcefully" implying with a great deal of energy or perhaps "powerfully"?