I'm only at four ERs. That's on purpose. I wait until I have enough unlock tokens to unlock everything between each level of ERs. I have enough now that I'll probably move up to five ERs today or tomorrow. I'm progressing at about the same pace but it is requiring me to use different attack strategies this season. Last season I slogged through with CRZM. Without cryos unlocked yet, I was running HZ for awhile but noticed that I was having trouble taking all four ERs out due to the high number of defenses (machine gun, mortar, RLs) that are designed to take them out. My opponents were using grens which I hate to use. I switched to them and now I'm progressing just fine.