Jan is not just a thief, but a caster too...
If Hide in Shadows doesn't cut it... cast Invisible or Impr Invi or best Mislead (ok this can be seen as Cheese
); if you do like that you can search for traps at the same time
LoS or Line of Sight tactics (with spells)
Jan -> invisible -> move up to enemy
a) take note of enemy position and retreat
-> move fire support (caster) up to enemy, but out of viewing range, place Area of Effect Spell like Fireball at the border of fog of war in enemy direction
b) (less cheesy) cast Mirror Image on Jan (this way an Image is destroyed, rather than Jan taking damage from friendly fire) haste is good too -> go invi; move up caster just outside of los, backstab, (pause) -> enemy dead = ok; else use your fire support...
the letter is standart military tactic (send in commando; if they are compromized -> take out the big hammer)