IMO any deck that features strong ELO clans can make 1300. Going higher, however, requires more precise planning, and a deck structured to fit the week itself.
As you may know, the ELO metagame changes every week. Keeps things fresh. So every week, you have to identify what cards you should be able to use, what cards your deck must have an answer to, whether there is a prevalence of certain abilities (Clover and Oshitsune free would mean taking T-Gaank over Dagg or X-0DUS), the rarity/commonness of SoA and SoB in a particular week, so on and so forth.
I know Leviaton is always on that list but he's not really a difficult match up. My problem lately has been that if don't win the first round (without grossly over using pills) that I've almost always lost. Do you all ever feel like this? I love elo, it's way better than other modes being as it seems more skill based and less like bingo but it's pretty hard.
It's not so much about a card being OP as such, but more about what that card does for it's clan. Leviatonn being unbanned just means that you're more likely to be seeing GHEIST. Cards like Marina+Saki, Bogdan+Olga etc. are very good, and will make their respective clans a lot more popular for the week. Those are just a couple examples.
You can always post a deck on the forum and people will make suggestions. If the deck is good, it's just an in-game decision making issue, which no one can help you with. It's just an experience thing.