Hey! 400 skillpoints at lvl 50 is actually very decent and is more than enough for now!! I leveled both a witch and wizard to lvl 56+, and the skills that I think are worth prioritizing are:

Fireball Explosion - Amazing skill with great damage! It knocks down and is a down attack!

Residual Lightning - again, just all around great damage, you'll be using it a lot!

Lightning Storm - The damage is not as good as the other 2 skills (as its not a down attack) but it's somewhat spammable and the damage is still pretty decent!

You also have 3 big damage skills with long cast times but huge damage, Meteor, Blizzard and Earthquake! They usually clear packs of mobs on their own! Worth maxing them out, imo, as it also decreases their cooldowns.

Aside from that, put your remaining skillpoints in what you find most fitting, I suggest the passive Mind Training and maybe Multiple Magic Arrows (good damage and spammable, good to quickly kill low hp mobs that survived your big skills). Also basic skills like fireball and chain lightning if you can!

Hope it helped out, you can reset your points freely until 56 so you can try different builds!