I may say he arrived through the portal before the dead Keeper moved in and edit that post if you don't mind. That'll make much more sense.
I'll say he hid in the south and then moved up further north when the old keeper died but before the wards were created, yes? No... I won't say that - doesn't even make much sense. I'm sure the portal was probably connected to the Northern chamber before Keeper A's arrival in Asgard. Nothing says it wasn't. He'll have lived between the opposing factions minding his own, happy til they came along.
The battle though... perhpaps the main last battle was after a smaller elite force of creatures had flanked them and already killed the Keeper. They tried to retreat back to the portal... but the elite force had already set up the magical wards blocking them. Over the next days they were trapped and slowly killed off by heroes from both sides. Any dead creatures in the dungeon could have been killed by the elite force before the creatures to the north were sufficiently thinned to make it back through and join the northern force in fighting the remainder of the creatures. The battle is finishing up as Stravos arrives. The last of the creatures are finished and they send a small detachment to checkthe state of the dungeon and that the wards they established to block the creatures retreat at the start of the battle are still in place. Meanwhile, the hero army heads back to wherever they are based and regroups unaware that their patrol have run into trouble. The dungeon battle last long enough for them to be gone by the time the survivors get away.
MeinCookie's possible timeline
-Flint arrives through unused portal and makes home in Northern Chamber
-Old Keeper sets up shop in this new realm.
-Old Keeper builds a few rooms and recruits a few creatures.
-Heroes notice the Keeper's presence and builds up an army.
-Several skirmishes occur.
-Old Keeper falls, special hero forces go behind main force, ruin dungeon and set up wards.
-Final battle in the northern chamber continues as desperate creatures are trapped.
-Hero south force moves north for whatever reason... north force needs reinforcement, simply to flank the dead keepers creatures or they finish of the last creatures in dungeon.
-A day or so after this fact, Stravos arrives.
-Last creatures killed, battle ends. Hero force sends a small force to check dungeon before moving out.
-Rest is history.
That would move the battle forwards a little. Makes more sense to me. Just a possibility.