In my opinion, smash not power is due to technique concern. correct technique give you more power; incorrect technique in smash, not only have not enough power but also possible injure your arm. if you have correct technique in smash, then need to do workout on wrist.

to gain your weight, you need to know how much calories need per day. then you eat more than you needed calories, you will gain weight. if you eat lean and workout, you get muscle mass; if you eat not lean foods, you gain fat as weight.

so, most probably what you need to do is EAT.