would I be correct in saying , to advance my encounter slots I would need to check which encounter slot I am on them battle all the pokemon up until the pokemon before the encounter slot I want.
e.g I start on an encounter slot of 4 and my pokemon is on an encounter slot of 7, would I battle pokemon 4 , 5 and 6 then the next battle get the pokemon I want?
Nope, that is not how it works.
For a frame, which Pokemon you battle will always be that of the slot which was reported in RNG Reporter. Having another battle just brings you to a different frame which will have a different encounter slot.