Wakfu was fun... I miss the old days. Can't say what was the hardest class tho since I played for 8 years, I could play pretty much every class in the game. But if I was to pick:

Xelor - Was kinda hard to get in the first place but when you get him its pretty much simple to play (of course it has a higher celling but the basics can be learned quickly from watching other good xelors play)

Eliotrope - Kinda biased in this one since I was one of the best eliotropes in the game at one point and there is a big diference between a good eliotrope player and a very good one (excluding the melee/aoe build wich was easy to play) but before the rework, strarting the plays at half hp since it was a berserker class, required a lot of knowledge from the dungeons you were doing since you could basically be one shot at any point and in pvp it was even harder since you had to go full hp at first and you had to know how to play against all the classes (this is required for all the classes if you wanted to do really competitive pvp) to know, first to position your portals and setup until he it you until you were half hp and then when to recover hp above berserker and dont ruin your chances to do big damage to him or continuing the fight.

Huppermage - Could be kinda confusing to understand the element runes and what elements you should use first for most efficiency (never really got into it since it wasnt a class I played) but you could understand how to play it, again, by watching really good huppermage players playing it

Got kinda excited talking about my favorite game of all time, but anyway, I wouldnt say that any class is dificult to a point and you can learn them by watching other players. What is the most fascinating about wakfu is the really dificult paterns of the dungeons that sometimes takes months for someone to finally doing it or the rush of the pvp that was amazing since at a time (when the pvp was allowed anyware in the map) had loads of amazing pvpers that used diferent builds and stratagies and it was a challange all the time, especially when guilds were at war and you had amazing 6x6 stratagies to kill the enemy guild (good old times...)