MikeP999
The balancing in casual is still not ideal. Regardless, the level of the opponent only tells you how much (s)he's played the game and doesn't tell you much about the skill of the player, although there is a correlation between those two. Once you've reach level 10, you'll unlock ranked play and then you also get a rank besides your level (which resets every season). Ironically, the ranked mode is usually easier than casual for beginners.
As for Imlerith: Sabbath, you will quickly notice that there are a lot of decks that have threatening cards, if you don't have an answer for it, your chances of success drop quickly. Instead of trying to remove Imlerith, you can also try to play a strong unit that can survive his duel. With time you will learn the decks the opponent plays and how to counter them. If you want some more help, you can ask questions in this thread:
GWENT: Community Q&A Thread - Ask Your Questions Here