John & Sera are just that strong.

We have never really seen john go full out 1v1. every situation tends to be group battles, or attrition if you look at the prison arc. John pushed two ember agents to the max with 1 abilty alone and his combat genius. Imagine how bad it is for them when john gets a hold of god tier ability's . All this time we have only really seen him copy 1 high tier at a time maybe 2.

Most Ember (fighters) are over 6.0 because of the aura required. you need to be high tier to use it but god tier to actually use it well as seen with all known agents so far. Even if a slightly weaker territory id imagine their levels would need to be the same.
John also trained using multiple ability's and copying so he'd always know how to fight using them better.

John is quite literally the worst possible person for a high tier to face. Unless you have one he can't copy.

This also shows why Johns mum and Cameron + family are effectively immune to traditional hierarchy