Just use 1 Character in a Fight. With at least 3 Enemies (more is always better) you have enough Room to run around, break armor, reduce hp to a 1 hit kill, only after that, start killing them. Ranged Units have an advance, but its also pretty doable with Melee Chars if there are not many ranged Enemies.
Bring Summoners to half or lower Strengh to get more enemies and Renown

I finished this Game with 4 leveld Units. Egil on lv 5 with Calabilla, makes him night Immortal, Rook lv 3, Allete lv 4 and the +3 Armorbreak item and Yrsa lv 5 with an Armorbreak item.

If you level up, get armor and exertion for Egil. Strengh, Armorbreak, Extertion and Willpower for your Archers.

In the Hard training Fight for Archivment, use 3-4 Heroes as Cannonfodder and kill them off, place Egil in the Middle and snipe with Nid/Eyvind in the Corner.

In the last Fight rush with Egil to the Front and let him tank Bellower and start breaking his Armor as fast as possible

Rook solo and +lv2 is a Monster with his 4 Movement, Ranged Attack,12 Strenght and 3 Armorbreak, no wonder they prefer him dead