You could go at it with the skill replacement option from thaumaturgy + some type of sponsored magic (even self sponsored, but you'd need thaumaturgy at evocation speed). Then it would just be a might replacement spell, and the rest would be using the usual grappling rules. Maybe adding a number of shifts if you want to emulate strength powers for additional stress.
So you could make a 4 shift might replacement spell and add 2 for "inhuman strength", so your target would have to go against a strength of 5 and you could inflict 2 shifts of damage per exchange. Though that might be a bit much, a high level wizard should easily be able to muster up +6 shifts to emulate mystic strength, and that will get deadly real quick.
If you keep to the simple might replacement, you will still get to slow them down quite a bit. And you can use high numbers to split them into multiple grapples, that should be ok. Though your wizard will need to repeat that spell every exchange, so he might run out of mental stress boxes real quick.
Edit:
One other idea, that might be a bit strange, but bare with me:
You could do a thaum-evo spell that replaces a resources roll to "hire some goons". Those goons can take the form of tentacles, clouds of smoke, whatever, that grapple the characters. And they should be able to keep up a grapple longer, without the sorcerer having to spend more power.
Edit2:
You could also create zone borders, that need various skills to cross, and if the threshold is not met, the character in question will have to take the remaining shifts as physical stress. The zone borders represent the various minions running around, trying to stop the PCs. The Players could try to eliminate some of the borders from afar. Imagine the final scene being split into something like a chess board, and you have your zones. They don't have to be quite that straight lines, but you get the idea. You might want to create some markers to indicate where everything is, depending on the size you chose for the scene.
Since large numbers of mooks can become pretty tedious to roll, you could have the players dodge against a fixed number if they are standing in a zone that has a "minion border". Increase that value by 2 for each border that is covered in minions.