In Marvel War of Heroes, as in all trading card games, people over-complicate boosting. They start to get into the deep math of it and pretty soon you have mathematician figuring out exactly how much every level costs on each card. Personally, I don't care for the long winded explanations.
Boosting Levels 1-30:
The easiest and cheapest levels to boost, I recommend players use common cards found in Operation two. When boosting in this level range I recommend beginning by using any cards that are not of the same alignment that are found. After you run out of these cards, start using cards of the same alignment.
The reason for this is that at lower levels, it is cheaper to boost. At higher levels, it becomes more expensive. Cards of the same alignment offer a higher boost. By doing it this way you reduce overall silver cost.
You should also try to boost 10 cards at a time. The reason for this is if you do it only a couple at a time you'll experience every price hike as they happen since at each level is a price hike. Bypassing levels, bypasses price hikes.
Boosting Levels 30+:
When boosting cards in this bracket it can get expensive. The cheapest way to do this is to first fuse all the cards farmed from Operation two and then using them to enhance. Although fusing the cards costs silver, it is less than than the amount per card at level thirty or higher and it will allow you to better bypass levels, thus bypassing price hikes.
(For this I recommend fusing the uncommon cards in these early missions to their are forms, the boost is huge and this way you can sell all the normal cards you find to offset. This will take longer but will save on silver.)
The same rules of using any alignment at starter levels and same alignment once you run out of them.
One you start hitting very high levels boosting in SSR/UR cards it becomes worth it to start using ISO-8 if you have it. It saves you a fortune and ensures three levels. I normally recommend you do this on the last ten level home stretch.
Don't waste time boosting cards you're not going to use.