[X] Total Magical Saturation. Two mage. One, the last scion of the esteemed Animusphere, and the other, the greatest of protege of Circe. Both enacting a ritual bolstered with five Command Seals. Such profligate expenditure will break through the vaunted advantage of the Knight Classes... but it would mean to deprive the two Masters of their capacity to react to sudden surprises.
Not sure if this will work but I think it's still better than basically screaming at Goetia's (well, Flauros') face that one of Chaldea's Masters is an immortal as the other option is liable to end up.
If writein plans were a thing Id probably feel something along the lines of "Use *some* seals to power the mages in an effort to lock down saber and restrain her for long enough that you can dash in full speed with rule breaker. Indirect magic to get around the resistance somewhat and to avoid needing to properly toe to toe salter by taking advantage of vampiric speed as a surprise factor +restraints
David has a line for pretty much exactly this:
Half-measures wouldn't suffice.
Attempting to restrain Salter in this manner should not work. In F/SN, Medea tries just that against both Archer and Saber on separate occasions, freezing the space around Archer and holding Saber with the air around her. Those attempts restrain the Servants for exactly as long as they allow them to.
If Medea could conjure up a construct/familiar physically strong enough to hold Seiba in place, it would be a different story since MR does not dispel such (point in case: Archer's projections) but any constantly running thaumaturgy will shatter like glass around her.
Those two instances form a dangerous precedent: Medea underestimates MR so if such a plan is suggested. in character she won't have anything to object with unless she has a clear enough recollection of F/SN to remember those failed attempts.