Strategy guide for the First Seal
Detailed monster stats are here
There're 2 little monsters beside the boss. You must kill the little monsters first in order to damage the boss. The boss hits for around 1400, so the team's HP should be at least 1400.
The boss' CD is 2. If after 2 turns either of the minions survives, the boss will attack you. On the other hand, if there's no more little monsters beside the boss after 2 turns, the boss will not attack, but instead revive the two minions which will start attacking nex turn.
It's recommended to initiate attack on all enemies in the 1st round by dissolving 5 conjoined runes at once, then focus the attack on the boss the 2nd round.
If the little monsters can't be finished in the 1st turn, try to kill then in the 2nd turn. Then the boss will summon the little monsters and you're assured that you won't be attacked and can wait for another shot.
Include as many cards which have multi-hit skills such as witches, starter cards (Sean, Molly, Duncan, Nathanial, Endor), defensive dragons etc. as they are able to wipe out the minions easily.