Slitherine

Hello: i am stuck on the Ork Expulsion final scenario in the Ultramarines campaign. I do well with Hellfire dreadnoughts and achilles landraiders, with terminators to take victory hexes, until turn 30. Then i am hit with a wave of brand new ork armor and heavy inf that wipes me down to an ineffective force. Their seems to be no end to the orks and not enough time despite the 40 turns. Any hints from those who have completed this? Thanks for any help.

Ps - this is the open field scenario choice
for the final battle, not the hive scenario.