There is a difference between a level 5 tandis and a level 3 Slough.
Tandis would have a base damage of 13, +5% crit, and 23 hp.
Slough would have a base damage of 12, poison 8 (base 5 +3 for level), 27 hit points, and no crit damage.
So, Slough is going to be tougher well before equipment, doing more damage overall because many units don't have a resist too, and have more HP. Not all heroes get a bonus to their base damage, Slough does not. They usually get some sort of damage increase (Ifris doesn't get more base damage, but does more fire damage for example). So, slough has a base 20 damage and more hit points at 2 levels below Tandis, who also has a lower overall accuracy.
Korthox has to be the worst hero though for damage. He get's a base 3 and 2 cold to start and only levels +1 cold per level. He has low HP, decent defense (one of the highest, but not the highest), and that's about it.