Alright, I made Mephistopheles killable, but he's gonna be TOUGH to take out. I'm wanting it so that your target shouldn't be Mephy at all, but Faust. Also, I added in the Ult's description that Mephy is visibly tethered to Faust's shadow (this meaning that even a novice could be able to see the shadow line connecting Faust and Mephy, thus being able to trace it back to the champion so they can find him and kill him).
I exchanged the unkillable trait with a passive increase to Mephy's armor and magic resist based on the amount of time he remains active, with the max being 10 seconds. Also changed Mephy's base attack damage by 50, but made the increase for each subsequent rank in the Ult as 75. I gave Mephy his own stats (minus health regen, mana regen, and spell block which he has none of as well as move speed which doesn't change with boots items). Overall, Mephy's only leveling capabilities are in AP (which is the same as whatever Faust gets), physical damage (already mentioned before), certain resistances (already mentioned) and of course he has a limit to how far he can move away from Faust (which isn't even as far as the range of TF's wild cards).
Also, "Merchant of Death" was a moniker given to Faust due to how dangerous his wares can be (his actual profession is selling illegally acquired items, even those that can curse or otherwise kill the customer). Also, he's a merchant by trade - acquiring, selling, or even trading various curiosities.