Good guide from Crimson Drifter (I voted positively on his before posting this). I will add that it is much easier with 4 players. Find players by creating a session here on TA. Create a Private Match for Campaign mode, start at the first level you need to complete on Ultra, pause it at the cutscene with the Start button, and then send game invites to the Party, wait for them to join and then resume the level. After the level is complete, the story will continue or you can back out to the menu. Four is the maximum number of players for co-op campaign.
1. In our co-op session, we found we had to back out and re-start at the current level every four levels or so because the lag would get progressively worse on each level for everyone except the host.
2. You can't die in Act 1 Scene 1 (Training) if you play by yourself, so you can skip this level in co-op and just do it on your own either before or after the session.
3. You can check if you've completed a level on Ultra at the stats screen for that level.
Also, here are some strategy tips from the developer:
1. If you are playing with a buddy online, try to coordinate what units you are. One
killer combo is the Destroyer and the Dragoon. The Dragoon can hide inside the Destroyer,
protecting it from nearby enemies and shooting down enemy torpedoes, allowing the Destroyer
time to bombard the enemy with its huge explosions.
2. If you are about to die, take cover and wait for your health to regenerate.
3. When trying to take an occupied enemy Control Point, swoop in with a fast unit type like
the Corsair or the Marine and drop all your depth charges on the enemy. When they die you will
have enough time to neutralize the Control Point so the enemy can't spawn there anymore, allowing
you to quickly take it.
4. Speaking of the Corsair, while it not as strong as the Dragoon or the Destroyer, has an
incredible rate of fire at a long (and relatively safe) stand-off distance. Try to find pinched-
off areas where the enemy will bottleneck their forces, keep a steady rate of fire on that
section, and watch the kills roll in. Even hitting the wall next to an enemy can be effective;
Corsairs and Destroyers do a lot of splash damage!
5. Remember that Undertow doesn't happen in space -- gravity is still a factor. There are serious
advantages to having the high-ground in a face off.
6. You can't shoot while using Turbo (the X button), but you can use depth charges. Zipping by a
pack of enemies for a bombing run is a good way to get some kills. You can hold up to five depth
7. Control Points don't have to be yours to get a shut-out. Sometimes taking a Control Point to
neutral can be advantageous if one is simply trying to deny opponents from spawning. Taking all the
Control Points to neutral, and then killing all your enemies (before they can get another Control
Point of course), is a quick way to win a match.
8. Use cover. There are areas the two smaller units (Marine and Dragoon) can hide from view. This
is a great way to take enemies off guard. Hide in tall grass, behind pillars, buildings, or in clumps
of rock and debris.
9. The five circles at the top represent the Control Points, and they're laid out from left to right
according to where they are located in the map. You can see their timers counting down when you're
taking a Control Point. The more team members you have taking a Control Point, the faster you'll
10. Upgrades are immediate, making you faster and more powerful; they also fill your health bar.
Remember that during battle! If it's late in a match and you're getting owned, remember to check your
points. If you have points, use them! Press the Y-Button to enter the upgrade menu, select the unit
you want, and then press the A-Button to upgrade.