This really can be considered as one of the most evident bugs in v2.3/BW - even Valdis isn't level 45 in mercenary mode...!
Also due to some proprietary extra game rules (about damage stepping depending on level difference) it's almost impossible to fight/win against a monster that is 20 levels above you.
Indeed you could win against the Undead Archer nethertheless without using cheats nor mods - just get rid of all the level 28 monsters first, then distract the Archer with a 'uselss' char/pet and run some moments later with a death/lighting/ice mage behind a wall where the Archer can't see you and then attack him from there - usually he won't looking for you.
Of course this is also a little (AI) cheat, but it's still hard work - you will need much mana and in particular a lot of time since your spell damage will be reduced down to ~10% by these extra game rules.
Actually there is a remarkable number of these hasty errors in v2.3/BW
(...and this post is already one year old) - missing set labels, botchy code rows, wrong armor requirments, ...
Honestly, it's not really grave but for experienced players it may become more distracting because through the hole game you may encounter these things that obviously shouldn't be as they exist. That's why the Aranna Legacy
mod will fix most oft these errors (although it never was intended as a 'patch mod' or so).
Supposed now you are familar with installing mods it also would help you with the missplaced level 45 archer.