Does the attack kill your Digimons? If it doesn't then it shouldn't be too hard to deal with its damage with a good healer and SPD buffer/breaker. Stack two sleep resist equips on your Digimons (if you don't have the DX version) and it should make them immune to sleep.
If the damage becomes too high then you can use INT breaker moves on the boss to soften its attacks. Have your buffer/breaker just use it's support skill throughout the battle to stack them and keep them up. The healer should just heal and guard while using buff skills or items on Wargreymon; either SPD up to decrease his recovery turns or ATK up to increase his damage. Doing this will whittle the boss to death.
If you're still having trouble then you could defend on turns that the boss uses its
since it usually buff itself before using it if I'm not mistaken. So use that as a tell.
As a general tip for hard, stat breaker skills are really good since they reduce so much of the opponent's OP stats. It's better to break their stats rather than buff your own party, though if you can do both then it will be even better. Made that discovery myself when fighting a boss in Chapter
; winning by essentially breaking the boss' ATK and SPD.
As for your other question, yes, the story does starts to pick up then.