General comments... unless you run the game in a one-way-to-go plotlines, then it's not on you to fo more than setup the "how do we meet" event.
I would suggest it be some form of raid by numbers of peons (give the low guys time to shine) then a bigger boss that needs the two big guys while the others help finish minions and help folks.
That ought to get to 2nd level. Then its safer for the newbies to hang and also they have seen the newbies eork.
Then it's up to them. Do they take a lesser mission to "season the greenhorns" or do they jump back into the thick of things risking the rookies?
Make sure they have ootions and challenges either way and in between and ***let them choose.***
That said - I use come back same level these days.