20 Turn 100% -> 84 CP 16 max mana Simple maze with a lv 90 boss. She can move twice, so be prepared you have characters that have 40-50 defense, or over 200HP for low defence character, else they are mince meat.
This boss will not move at all and you could snipe him using any ranged attacker. She has a low SP regen, hence after a few turns she will not be able to retaliate any ranged attack, leaving her harmless for a few turn until she regen enough sp for another one ranged attack.
There's also a non-boss version of Helte, the naga angel in EX-02, you could catch her here normally using Lily's capture skill ( 捕獲攻撃 ) or 捕獲攻撃・武器 from 蛇王女のロープ.
There is two hidden room adjacent the room with 3 mana towers, on on top left area, one on bottom left area.