Is this Time Weapon too good?

Hi,

TLDR: I am making a time weapon that might be too powerful?

There is an encounter my party will soon run into that's a village in a time loop. At the start of this war some of the Bads tested their new Cannons out on this village and a retired time wizard who happen to live there resets the day in a panic, every day.

This encounter will be oppressively hard, but the party get to keep having a go at it and can fix what went wrong last time, they know where the enemy will be and they can study/prepare...

However, I also want to have an item that can help them win, but that they don't want to use. Something as a last resort that they will only use if they can't get out.

Idea so far: A permanently binding ison stone which whenever the user attacks, they roll the dmg it will do, THEN roll to hit. If they miss, they auto-rewind time a few seconds and hit again (re-roll their to hit), but if you still miss, that dmg happens to you. This is on theme but is maybe too powerful??

What are your thoughts and/weapons please? Or do you think this is a flawed idea?