hmm, the case when we were ambushed by them was like this:
- killed them
- went into crypt
- came out 30-60 minutes (or however long) later
By the time I loaded out (I load slow) they were already mobbed on the faster loaders.
Since the giants perception range is so long (as far as a dragon) this is probably a spawn trigger issue (doesn't matter if the spawn point is far enough, if you hit the trigger as soon as you come out they'll rush you from really far away.)
The spawn trigger should not encompass the positions immediately out from the door. In fact this is kinda tricky to handle since their LOS is so long, anywhere the trigger is coming out from the crypt will let them see you as soon as they spawn. Maybe have them only spawn on the way in? or face the crypt 180 degress the other way, so that the crypt is between the spawn and you when you exit giving the party a chance to all load in and get organized before the upcoming battle (since it will block their LOS untill you round the corner of the crypt building.)