Leveling up my most used cards to level 2

Definitely a good move.

What I did back then was focus on EARTH and WATER splinters. I'm not sure what summoners you use but if you can at least level up your EARTH summoner to level 2 then level it up. Also if you can I highly recommend the Water BETA summoner instead of the UNTAMED summoner.

The main reason as to why the EARTH BETA summoner needs to be at least level 2 is that so you can level up your FLESH GOLEM (EARTH monster) to level 3 and get that "heal" ability. When paired with the Earth monster "Wood Nymph" who also has a "tank heal" ability, you get yourself a super tank which can deal with most opponents who uses around level 1-3 summoners.

For the WATER Splinter, if you can get at least a level 1 "Ruler of the Sea" monster card get it. With the BETA WATER summoner's ability to increase magic attack by 1, the "Ruler of the Sea"'s "blast" ability is a beast in dealing with opponents who also uses around level 1-3 summoners. XD

That's still pretty much my strategy on my other alts. XD