
Zautos' |

So a level 10 character can cast this with a 4 level spell.
the DC could be
10+4(Spell level)+6(Int)+2(Greater spell focus)=22
If the target tries to cast a level 5 spell the DC would be 26.
If the target is level 10 as well it would have a bonus of. 10(Level)+6(int)
So a 50% chance of making the save.
I'm guessing that the chance will stay around that at earlier or later levels.

avr |

Yeah, but the spell also has to affect them, and of course a solid mid-level spell which affects a spellcaster may well be able to stop them casting spells anyway. I'm not saying it's useless - a spell like disruptive ice storm might be useful in the right situation - but you might find that a hold monster spell or something would work better.
Also remember that there are items which can boost concentration checks. They're rare on NPCs, true.

Zautos' |

Yeah, but the spell also has to affect them, and of course a solid mid-level spell which affects a spellcaster may well be able to stop them casting spells anyway. I'm not saying it's useless - a spell like disruptive ice storm might be useful in the right situation - but you might find that a hold monster spell or something would work better.
Also, remember that there are items which can boost concentration checks. They're rare on NPCs, true.
Looking for a meta-magic feat to add to greater dispell magic. I did not find many that are fun and good :(