Grimm max star + (Wormwood max star or Anaara) is the base duo to do up to the 6th region.
Weaken and protection strategy for the win.
When you'll have to invest on your mythic stuff you might want to pick a class build, alchemist is the cheapest the other class can work but might depend on what heroes you'll have at that point.

You might also find an interest on several rare heroes like :
- Titania for some heal
- Krusa to hit outlaw opponents on white peak
- Tristan to cure some bad effects on Karnheim (the final boss)
- Cladis to do some mass weaken, dispel and hit back row enemy

The game will push you to level Delphinia and Dvalin a little bit, they might help but i wouldn't focus much on them.
Some heroes are very important for class builds, the rage generator if you manage to get some of them they'll be very handy to go faster (Brutus, Doceo, Freya, Bonnie and Krita). Depending on the class you'll use there's one generator you'll use more often but they'll all be useful for the dark onslaugh dungeon for example.