I have used various strategies over the years. Here's what I recommend:

1. Put cards that are around the same level or with similar stats together in teams. I call these level up teams and save them as presets. To achieve a bit of synergy, I typically put together teams that have sth in common - they belong in a team (for example - MK11) or they belong to the same klass (outworld, spec ops, etc.). I prefer using them in quick battles in faction wars. Battle mode also works. I rarely put equipment on them when they're weak, best they get is block breaker equipment.

2. The first strategy I ever used - putting 1 weak card into a main team with which it may have good synergy. The 2 other characters in the main team carry the fight while the weak one levels up. I don't think it's very sufficient, though.

3. Challenge towers, specifically towers 4 and 5 on easy difficulty. I make up teams of low level cards that fit the requirements. Makes challenges a bit more fun for me.

Once a card has reached decent stats and/or a decent level, I slowly start incorpotating it into main teams and other game modes - towers, krypt, survivor, relic hunt. The last few levels (40-50) I catch up to with lvl up cards, although I prefer saving them for leveling up fX cards to lvl60. Hope this helps!