The plot would be about Mephisto using guile to use the Stranger to augment his realm's power, or for some reason coveting the Stranger's soul. I don't think he would go toe to toe.
But Mephisto made a pretty good showing against Galactus for a moment, before Galactus started eating his entire realm!
The Stranger once fought Pluto (the Greek god, not the dog), in Pluto's Netherworld (this was in an old Marvel Two-in-One Annual).
The Stranger appeared to be struggling because he could not replenish his power while Pluto could continually draw on the power of his realm to recharge...
During the course of the battle Pluto stated that on neutral ground the outcome of the fight would be in doubt, but that in the Netherworld the Stranger's attack was "laughable". However Pluto later admits to himself that he is sorely tested and has had to draw more power from his realm several times while the Stranger is still battling on with his own natural power...
The Stranger eventually feigned defeat to further his plans.
It's interesting to compare this to the Galactus/Mephisto battle where Mephisto eventually conceded defeat even with the home ground advantage... but that was because Galactus threatened to consume his realm.
As death-lords go, I don't think Pluto is as powerful as Mephisto. As a cosmic, I don't think Stranger is quite up there with Galactus, neither does he have the world-consuming attack up his sleeve.
So I'm going with:
Neutral ground - Stranger repels him 9/10
Mephisto's realm - Mephisto takes 6-7/10.
On the other hand Hudlin Panther beat Mephisto for god's sake.