Should i level up Caster Medea ?

If you like her or you like the idea of an arts team that spams NPs then yes, if you don't then no. This game is easy enough that you can use whoever you like and still do well, and it is forgiving enough that if you level a "bad" servant you don't want to use, it won't really matter in the long run.

She is a good offensive caster, and one of the best servants in the game at spamming her NP. Since she is 3* you can easily get her to NP5 making that NP spam even better. She is really good at that one role, but not much good at anything else. Thankfully that role can be really fun/effective.

At level 40 I feel she out performs caster Cuu chulain who I have at 60. It wasn't a perfect measurement since it wasn't terribly difficult content, but the highest level ember gathering got way easier when I switched him out for her. Her survivability is PROBABLY a lot worse, but she does deal damage faster, and even if not for removing buffs, her NP can come out fast and often and provides a good spike in damage.

Considering KnK will likely release after Valentine's. You should consider leveling her. The KnK event will have a lot of assassin mobs.

Medea is always a great caster to use. Her NP is great not only because of her being able to spam it with impunity with the right setup but because it removes debuffs. Because of that, Medea is a top tier choice even for those who have gold casters. She can also remove an ally's debuffs with her 3rd skill once the quest to unlock it arrives.

Her main weakness is her lack of defensive options outside of her self heal. While it does have a low cooldown, against enemies who can deal consistently high damage, it won't be enough to keep her alive.

The best teammates and CEs for her are ones who can keep her in the fight and ones who can help her NP charge up. Her true damage potential comes from being able to spam Rule Breaker so that should be your focus rather than direct attack buffs although they are still useful.