I only just started playing for the first time last week or 2 weeks ago, that mantis boss was tough but i managed it after many attempts. Each time i returned i played better. Little tip for you, in the arena, before starting the fight, wall climb up the left hand wall and attack upwards at the ceiling a few times and a hole opens with those blue blobs that give you temporary extra hit points. They respawn if you die so you can always start the fight with a few extra hits.

Only other tip is remain calm and the fight will feel slower.