Knowing there were Space Marines aboard this ship changed the tactics completely.
Peter issued his first order:
"Thor, the mission remains unchanged. Secure the engine room or destroy the engines. Call in Third Squad for reinforcements if necessary."
No matter what, the heart of any ship was always its bridge and engines. That would never change. That was the nature of battle: plans never survived contact with the enemy, so all they could do was trust in Thor's squad.
Their own target was the bridge, the key to controlling the entire warship. They had to move quickly and decisively, too, because any delay could lead to the boarding action's failure.
Peter continued giving orders:
"Form a Shield Wall."
Two shield-bearing members immediately stepped forward to form a Shield Wall with Peter. It was a shame that one of their shields was a Combat Shield, not a Boarding Shield.
It wasn't that Combat Shields were bad, but their defensive capabilities were inferior to Boarding Shields. Combat Shields were a compromise between mobility and defense. They were small enough to be mounted directly on the arm, freeing both hands to wield another weapon. The brother carrying one had a Bolter Pistol in the hand holding his shield.
Now that they knew the enemy had Space Marines, Peter had to move faster. So instead, he raised the Shield Wall and advanced slowly. It seemed counterintuitive, but it was the right tactic.
When facing only mortals, they could naturally stride forward, relying on their Power Armour to shrug off small-arms fire. Even against the occasional heavy weapon, they could deal with it first thanks to their faster reaction speed. After all, there was no room to dodge in the corridors. Whoever struck first had the advantage, and Space Marines were usually faster than mortals.