When the Brace phase starts, take the opportunity to identify the next incoming enemy leader, bonus Mythic Heroes, bonus structures, and make preparations accordingly. You can also do this during Shield phase, so there's no need to rush.
During Shield phase, you will be victorious if you can defend Midgard's Shield from the enemy for seven turns.
You will lose if both of the Gateways at the bottom of the map are destroyed.
Your summoner is now part of your defense team in Mjölnir's Strike. You can customize the summoner in Allies → Interact with Allies → My Summoner.
Your score will be reduced slightly if an ally is defeated or a Gateway is destroyed. You can attempt the battle as many times as you like, so keep on trying to get the best score you can.
The members of your defense team can use Pair Up, with pairs arranged vertically in the team-editing grid. (Your summoner cannot be part of a pair.) There are also individual bonuses within the grid. The leftmost Hero will be placed at the top of the map and will not be the target of attacks from your foes, but they can still receive counterattacks, so be careful. The other Heroes will receive boosts to Spd, Def, Res, and Atk respectively.
Deploying a Mythic Hero who is currently under the effects of a season bonus will give them a boost to their stats, and they also increase your score if you win. There are major and minor bonus effects, but even the minor bonus can make a noticeable difference—so if you've got a Mythic Hero who can take advantage of the season, don't leave them on the bench!
The rightmost slot on the Edit Teams screen is a special slot that is activated by including a Mythic Hero with a major season bonus for the current season on your team. Only Mythic Heroes currently receiving a major season bonus, or Heroes who have been bestowed with a blessing matching the season, can be placed into this slot. Also, Heroes placed in the special slot will not influence your score.
About Mjölnir's Strike (Basics)
Mjölnir's Strike (Counter Phase)