Random encounter. R1
Pretty much same circumstance as jiraya versus pein
He's just dropped in there no intel
In character
In round 2.
In character knowledge
What minato would know from seeing the fight in naruto's brain and what nagato would know from minato's reputation
Nope. No sage mode
He only gets kcm in r2
He can summon toads but with pein's summon army not sure how that would fair. R2 only
No animals summons in r1
Hmm, you limited Minato in that case pretty well.
So. For the random encounter, I will still pick Minato even if it's my cope, Minato is a nasty and fearsome fighter and he wouldn't have to bother about protecting anyone. Like, having Kushina around is the only thing that can nerf Minato cause he cares.
He was a menace to the world during the war.
I don't know about the difficulty tho, he's one of the smartest Ninja ever, he would come up with a plan and after few tries he would understand Pain's main abilities, I think almighty push would be overcomed with the seal Minato would put on him in one of time splits between the use of this technique.
In other words, he would put main Pain down the way he did Tobi.
Round 2 is a huge boost to Minato, I just can't imagine Nagato winning.