This can be a frusurating acheivment. However, it could be a breeze as well. Common sence says use co-op. Find a partner. He doesn't have to be necessarily good. Have him/her play on Casual and your going to play on Insane. A basic rule of thumb is to keep two frag grenades on you at all times in case of a close quarters confrontation with a boomer or grinder. You should have a Sniper/Torque Bow (A Hammerburst or Lancer if neither is availaible) and a Shotgun (Ammo isn't hard to come by.) For a pistol, keep the Boltok as a preference, if ammo isn't readily availible, use a Snub. The Gorgon should be avoided unless you know a different pistol is near-by, as ammo drains faster as the weapon fires in bursts. If you kill a Kantus, don't let the Ink Grenades go to waste. Simply swap them for frags, throw them/it, and pick your frags up again. If no enemies are any where in the premisis, you may leave them.
Your partner should take the aggro is fire fights. This means he often exposes him self, and injures as many enemies as possible within a seven second window. In this window, when fire is concentrated on him, open up. Snipe out enemies by the danger they present in the current situation (Bloodmounts, Maulers, Boomers ect.) and wash, rinse, repeat.
Leviathan- The giant fish. Your given frags. And a lancer. One person shoots while one throws. Not that hard.
Skorge-Ground - Stay in cover and move around, to avoid inks, tickers, selaktites, and falling colleums.
Skorge - Reaver - Stay in communication with your partner and shoot at the same part a the same time and this will be over in no time.
Sires - This is one case where it isnt a bad thing to charge at the enemy and use a Chainsaw. In most cases with other locust, you'd be surrounded by the time the sequence finishes, then shot to death.With Sires, they have to melee you, so just rev up again, and saughter the monsterous Locust. Note - In campaign, any damage you take while chainsawing an enemy is reduced to 0.
Giant Riftworm - This isnt difficult at all, just keep a Lancer out and shoot the little white enemies. They die easily, and don't inflict much damage, even on insane. The most frusturating part is the area in the chamber with limited visiblity and toxic air. It is very easy to get lost, but Roadie Run on forward, and follow the tracker on your HUD when you see the locator for the membrane. On the part with the Indiana Jones style huge ball of death, don't touch the grass or slow down. It's not good for your life expectancy.
That's it!!! If you follow these tips (and other guides, as I clearly have forgotten some areas) you'll be sure to finish the campaign in under 20 hours.
(How come they let the queen just walk out of the throne room when they have GUNS?????)