Well a couple of weeks ago I was getting a nice return on single Cornealus and Scale Doctors, better than the maxed!
I actually bought another single Cornealus with my seasons rewards (which were shit) rather than opting for levelling anything up. I've kind of got most value cards levelled anyway.
Summoners I guess would be the other thing to go for. I think I'm just going to stack summoners from here on in.