Hi! This is how you can clear Mercia without using any items or any retries. I chose to use just 3 Heroes with no equipment or runes to show that this will still be a viable strategy in the higher stages.
Ensure that Kiera has at managed to lower her cooldown by at least 1 before progressing to the next stage.
The same unit will always be pulled unless you using a wide range of Friendship levels or change any stats. If the unit is unable to survive the pull damage, put a Miracle rune on him/her.
Move the pulled unit back until Mercia is about to cast her spell. Cast Kiera's spell. Cast Arthur's spell. End turn until the cooldown on Kiera is 1. Cast Tristan's spell. Rinse and repeat till victory.
This combination can be paired with 2 Rangers to help expedite the progress. I will prepare another video once I have reached a higher stage.