Welcome to the game
Minions are always great for autofarming, absolutely!
I´d perhaps swift them around a bit, so that Marcel is to the left, and fires first, and C Grimm fires afterwards. Then Frosty, Freya and Proteus.
It might be an idea to use a hard direct hitter instead of Proteus though. Mana block is great, but if it´s for autofarming, it´s usually in content that you´ve already completed, so it´s more about dishing out a lot of damage, than blocking the incoming damage, if that makes sense?
But again, it depends where you ‘are at’, in game. Bringing on a dispeller / cleanser is not a bad idea either, depending on which content it is you are farming and/or playing. It can also be very helpful to bring heroes that deal damage to all, if one wants to speed up things.
But in general, bringing a defense down, crit /attack up, one healer, and then dispeller /cleanser, and then you´re really good to go
If it´s map progress instead of farming, then Proteus helps so much, I´ve heard, though - so great pick for that.
If you are very very new to the game, those 5☆ heroes can indeed be very time consuming to level at that stage. But, Freya is amazing though, congratulations on her, you´ll love her
Have fun and good luck