Strategy guide for the Second Seal
In general, the enemies are weak and it's easy to kill them. But you should be aware of the Cursed Sword and Honeymon which are good at defense and attack respectively.
As for the Boss, there's a pair of bosses sealed here. Both of them need to be killed together before the CD of any of them reaches 0. Otherwise, one enemy can revive when the remaining boss CD reaches 0 (instead of attacking.)
This means if the if the CD of a boss is at least 1 or more, you still have time to kill the other boss before it respawns the other. Because of this it is highly recommended that your team has monsters which have active skills in attacking the enemy.
Both bosses can inflict 3000 Attack in total: Light - 1100; Dark - 1900. Unless you have killed one boss and they revive, the damage should be inflicted at the same turn for 3000 total. It's better to set a team with a HP of 3000 to avoid being killed by the boss in one turn.
A general strategy is to attack the bosses at the same time, either by using 5 runestones at a time or alternating between bosses to keep their HP relatively even.