I would just use all of your junk items that you don't use to power up the items you do use often. That way you'll tidy up your item collection and make use of the junk items.

Alternatively if you've already done that method then the best way is to fight online or offline to gain credits and buy the 5,000 item box's, any items you don't want just use the for power ups, doing it this way is more efficient than buying separate items for the shop as its cheaper and you get 3 items. Also you could find a legendary item that you might actually want to keep

Hope this helps,

Myles

in-game name: Timeup

Sadly buying normal boxes is the most efficient SM - Fusion converter.

It's noticeable when you buy in bulk, say, 50 boxes or more (250,000 SM Coins).