As a mediator main i tested a lot of things with the character.
Skill 1 : The first skill seems good when playing the character for the first few times however maxing it first is mostly useless as it has no horde clear whatsoever. The skill does a lot of damage to single enemies but nothing to a group of mobs, even with skill 2 activated as we'll see later. Upgrading skill 1 is usefull only to proc more on hit items.
Skill 2 : This one is really good at level 0 and completly broken at level 1, but the problem is mostly that for the first 10 stages it's almost useless since at level 0 it will do close to no damage, the target will die too quickly for the skill to be relevant and it has a rather long cd for a skill that can in some case just disapear after 1 tick of damage if you accidently put it on the wrong mob. At level 1 the problem is still here as you can't put it on high hp mobs since you use it to kill them and the skill doesn't do damage to the target, it does have a huge horde clear tho. Level 2 and 3 are not as usefull as it sound.
Skill 3 : This skill is fine as it is nothing to say about it :/
Skill 4 : the weirdest but most broken ability in the game. Starting from floor 1 it can already one shot hordes, rampant and even sirens if you play it well. Once you have the level max on this skill it becomes an entirely different beast and let's you insta clear every horde and use skill 1 to one shot every siren. This skill needs either a nerf or a rework as it is really gamebreaking right now.
Passive : I have no use for the passive since there's never any curse items in my runs :/