Originally Posted by ChaosLord29
I'm willing to cut the Covenant a little slack where Scarab's are concerned. I played the Halo RTS and let me tell you a Scarab Walkers tear their way through UNSC vehicles like tissue paper. Considering that 40k is basically the Dark Ages with Super Science technology, I'm willing to say the Scarab could pose a serious threat even to most of standard Imperial vehicles.
Problem is, it's the only serious threat I can think of, and Scarabs are pretty rare. Focus fire from a trio of Leman Russ Battle Tanks would be more than enough to take it down I'm sure (that much faster if they've got Basilisk support, which they will). It's maybe a match for a lone Warhound Titan, but seeing as how they're employed in scouting pairs and supported by the larger titans . . . it doesn't look good.
The Covenant Ghosts and Banshees might do a lot to harass an IG regiment's flanks, but seeing as how they can be gunned down with concentrated small arms fire . . . Well, they're basically open topped vehicles with an armor value of 7 or 8 (lower than even the lightest vehicles in 40k).
Concentrated Slug fire in Halo can kill a Banshee. A few shots from the Sniper Rifle, say...3, can outright destroy a banshee. I seriously don't think they'll be able to do much other than make extremely quick hit-and-run tactics. All the Guard would need to do is get a few solid Lasgun hits and BOOM! Instant dead banshees. The same can be said for ghosts, though they're in an even worse situation since they aren't airborne. I have far more confidence in Phantoms and Spirits being able to harass guard lines, of course this is assuming that there aren't any tanks, Heavy Weapons Teams, Sanctioned Psykers, or someone with a Bolter immediately available.
As for Scarabs, I can see its main gun being able to destroy a Leman Russ, but let's be honest: The projectile is slow-moving and easy to avoid, with a dissapointing lack of AOE to compensate. Guard and Tank commanders will eventually catch on to this, and learn to fire their tanks on the move. A Warhound Titan is redundant, all that's needed is concentrated Battle Cannon fire, Demolisher Cannon fire from a Baneblade, or a Basilisk/Manticore bombardment.
And I think you're forgetting the sheer over-the-top-edness of 40k. The Imperial Guard is NOT the UNSC forces. Each Guardsman is zealously devoted to the omnicide of all non-human sentient races, and equipped with weapons that put half of the UNSC arsenal to shame...and that's just standard issue. I have no doubt a single guardsmen could gun down an Elite without much difficulty (though the energy uzi might do some hurtin' too)
You are right in the OP when you said it's a blowout, but i'm not sure you truly appreciate how massive a blowout it would truly be. With the scenario provided, it's pretty easy how the battle goes. The IG holds the planet without much difficulty, the Marines send a single company, possibly a Terminator Boarding Squad in to destroy High Charity from within, and they succeed with the Covenant being powerless to stop them, and their fleet is decimated by a huge Imperial armada of ships that are built to deal with threats of far greater magnitude than the Covenant could ever hope to bring.