[Leeko]

I decided to replay War in Heaven, and much to my delight it's every bit as awesome as I remembered. Except one problem I don't remember: Delenda Est being impossible. I can't get past the second half. If I skip to it, the Deimos corvettes won't stop turning, and end up at angles where it's impossible for me to hit their beams from standoff distance, usually with their engines pointed toward the Frigates. When I play the mission from the beginning, they end up perpendicular to the friendly fleet (which seems like it's what they are supposed to do, correct me if I am wrong). I've been stuck on this mission for hours, and now that I realized the corvettes don't position themselves as asininely as possible if I don't skip ahead, I have another problem. When I do manage to neuter all the beams present (MUCH easier when I don't skip) the Katana and Altan Orde will be at somewhere around 5% and 15% respectively. Sometimes it switches, depending on which corvette I go for first, but I've had more problems keeping the Katana alive so I usually go for its attackers first. So then the second destroyer warps in, and the Katana gets vaporized. Game over, no debriefing. Every single time. What do?

[Ryuseiken]

Well I played through the campaign on normal difficulty, though I'm not sure which level you have yours set to. On Normal while skipping ahead when I failed I found that I just needed to immediately hit my burners to get to an angle I could take shots from. Then I just used V to target the beam turrets on the Deimos and used the Archer to take them out from far away. As long as you aim carefully it's easy to take them out and you just need to work on your speed. Then you need to speed over the the other corvette and repeat.

Ignore the fighters unless absolutely necessary and let your wingmen do the work of clearing out the enemy. Your Uriel wingmen also do an admirable job at taking out turrets on their own so I avoided giving them orders. Also, once you take out one side of the Deimos' beam turrets make sure to take out the other side's before they start to roll and get a shot off on the Katana or Altan Orde.

After you defang the corvettes just make sure not to ignore the cruisers and take out their beams as well, they're usually much easier to hit than the Deimos' so it shouldn't be too hard.

[Kolgena]

It sounds like you're playing on Insane. I also have the feeling that the mission is harder... At initial release, I managed to beat the mission on insane on the 6th or 7th try. Now I can barely even do it on Hard.

Also, skipping dialogue and loading checkpoints can actually work against you and make the mission harder, IIRC.