Fall back to the basecamp area where we landed our troops, deploy heavy weapons effective against power armour (multi-meltas, plasma cannons, Lascannon, missile launchers w/ Krak ammo). Keep the big guns in heavy cover with clear fields of fire to the entrances, and make sure there's plenty of extra ammo with them.
Remove all objects that could be used as cover by the enemy and set up an solid perimeter about 100ft from the entrances, leaving a large killzone where the enemy will be caught in the open.
Set explosives in the hallways the enemies could come from, and have our techmarines find a way to remotely seal the doors to said hallways, that way, when we see them coming, we can lock the doors and expose them to the void with the explosives.
Have the thunderhawks drop in plenty of extra AP ammo, such as Kraken bolts and/or vengeance bolts, plenty of melta guns and plasma guns, and crates of Krak grenades.
Find our 4 best marksmen amongst the first company, give them each 1 of our 7 sanctified bolt rounds, and get them stalker bolters if they hasn't already got one to use the bolts with, get them to hold fire and scan the foe until positive identification has been made on Ashkenor. Attain said identification with the assistance of Trianon and Damokleon, to know when and where Ashkenor is coming and have our snipers ready to take the shot. Keep the remaining 3 bolt rounds for ourselves in case we have to combat Ashkenor 1-on-1.