It depends on the criteria of how you are comparing the bosses- if you fight them all alone, without back-up, then Cerzak does seem easier to handle- apart from his speed-reducing attack, you can clearly see his stats and his area attack is limited to 3 tiles, whilst Nelis has two types of attacks, both of which are quite potent- one can kill a teammate in one go (if you don't train up enough) and damage all your teammates as well, and another is a 9 tile area attack- in that respect she is superior to Cerzak. Stats-wise, she has higher hp and regenerates more hp per turn, along with obsured stats, it's much harder to judge when she's nearing defeat, thus preventing the player from using a gang-up and semi-suicidal rush on the boss, which they could do if they saw clearly the stats (of course, upon the condition that the player wants to keep as many teammates as possible alive).
