The concept of stopping enemies for a time seems very promising, but there are a few things that I'm concerned about.
If projectiles and spells would be frozen by this effect, it's expected that the player would attempt to create a barrage of attacks that would only be unleashed when the effect is ended. Stopping projectiles in such a way might be bad for damage if the attacks are piercing. If my understanding is correct, a piercing strike may only damage an enemy once every 1/6th of a second. If a user with the normal maximum of 200 Mana could freeze time for 10 seconds with a loss of 20 mana per second, they could easily have a part of their strikes go past the enemy rather than deal damage.
While time is frozen, the user cannot use mana potions, but what about mana regeneration granted by buffs or equipment? A set of Nebula armor might be able to regenerate past the loss of 20 mana per second with their mana boosters. 3 stacks of the Mana Booster would result in regeneration of 30 mana per second. Perhaps mana regeneration should be canceled for the user that is stopping time.
Negative colors might be a bit unpleasant to look at. Some of the most dangerous NPCs can spawn in dark areas such as caverns or during the night, which would result in very bright colors when converted into negative. It would be a very sudden change in brightness.
Finally, would the game's sound be slowed as well? It might be more impressive if even the soundtrack were to be altered by the Stopped Clock.