Gentlemen, I am sure you all know why I have gathered you here today.
As we all know, the Dark Lord who has appeared on the borders of our beloved realm of Average Magical Medieval Setting commands his legions of demons and trolls with an iron grip and threatens our very existence as a cruel feudal society based on slavery with nonsense such as "industrialization" and "human rights", but what truly makes him worse than the seasonal run-of-the-mill insane necromancers is his recently patented defense spell.
For those of you who didn't get the memo, this incredibly slightly-stronger-than-average man is the inventor of a passive spell that is independent from his senses and, in event of imminent death or damage to his body by physical means, stops time from his perspective and warns him of the danger that set it off, allowing him to move in a radius of 10 meters and manipulate anything he can touch. As long as he does not exit the aforementioned radius, time will stay still. We have however recently discovered that the spell has a downtime of two seconds after every activation.
He has so far survived magical bullets travelling faster than the speed of sound, explosions set off behind a wall of his room while he was sleeping (I've heard he spent about five "hours" in stopped time before waking up and realizing something was wrong), ghosts that could punch very hard, Endless Pits of Death (did I mention that during time-stop he is not affected by The Invisible Imps That Push Everything Down, and he can thus float out of the way of danger?), and even our polymorphing assassins got squashed when they tried to inoculate him with lethal poison while disguised as mosquitos. And of course, stopping time has been trademarked as a vital part of his spell under planar copyright laws so we can't just send a couple of warriors armed with it against him.
So here we are. How do we stop him before everything we know and love is irredeemably made better for everyone else but us?
Yes, yes Arnold, I know what you're about to say, but not everybody can get mad glimpses of the future like you, and as I already told you earlier we don't even know what germ theory is, let alone atoms and nuclear reactions. All we can do is try and beat him only using something that makes sense for our setting.