English is not my first language, so bear with me.

There are some really good guides in Steam's discussion section rather than guide section, if you like to check them first:
1. Every companion's breakdown
https://psteamcommunity.yuanyoumao.com/app/617480/discussions/0/1646544348831671886/

2. Guide into Maximizing DPS, a.k.a. Godly DPS.
https://psteamcommunity.yuanyoumao.com/app/617480/discussions/0/3211505894137106184/

3. General Strategy for Min/Maxing
https://psteamcommunity.yuanyoumao.com/app/617480/discussions/0/3211505894149475815/

Physical damage is reduced by 1 point from each 1 point of armor.
Elemental damage is reduced 1% from each 1 point of elemental resistance.

In the early games:
20 physical damage - 50 armor = 0 damage done
20 elemental damage - 50% elemental resistance = 10 damage done
elemental damage wins early games, plus most early enemies are weak fire resistance

On the contrary, in the mid to end games:
1000 physical damage - 50 armor = 950 damage done
1000 elemental damage - 50% elemental resistance = 500 damage done
enemy's armor can be ignored almost

Physical damage or elemental damage? Which is better?
Generally speaking, when the damage per hit is above 100, physical damage wins.

Support heroes are: Kane, Aeric
Damage dealing heroes are: Maeve, Rakhem, Boron, Whisper, Kaela

Kane:
At level 5, Kane can get the VIP skill "reinforced armor" with party health regeneration path. And it can be set to auto-cast.
At level 5, reinforced armor can grant the whole party 4 armor and 3 health per second with 100% uptime. If putting most of kane's attribute to mastery, crafting even only 1 piece of mastery green gear, the reinforced armor skill will grant the whole party 7ish armor and 5ish health per second.

From this point of time and onward, the party should be invincible.

With some min/max, by putting on every mastery/skill power item, party armor and party health per second will be easily 20+, which is more than enough to carry the whole party to beat the epic difficulty game easily.

For the rest of your party member, check the links above and pick whoever you like.