Cant get in the Hollow Tower...

[uff]

Hello,
after killing all the enemis in the Moonrise Tower on the rooftop, we are unable te continue to Hollow Tower because the last Necromion is still listed on the enemy list on the top of the screen and it's stopping us to go further.
Any ideas how to solve this?
In the meantime we were doing some other stuff on the map, so the option to go back and load game before the fight with General Ketheric Thorm is unavailable.

Thanks in advance