Walk to the water and swim. Let all the ants gather around you. Eventually you can walk on top of them and they cannot attack you. Just run around and shoot at your feet and they will eventually die. You can do this on the Inferno difficulty setting and get the mega powered weapons early.
Level 25: Easy completion
To do this, bring a weapon of your choice and C70. It starts appearing on the Hardest difficulty around level 36; try farming level 40 on Hardest). As soon as the level begins, start dropping C70 off the sides of the path and detonating them. Remember that the C70 has a 40 meter blast radius; be careful not to be in it. You should be able to get the enemy numbers down to about 20 by doing this. Then, just mop up the remnants. Note: This also works on Infiltration 1.
Mission 21
This mission will introduce you to the big brothers of the black ants: the red ants. These ants have an incredible amount of armor, and attack in huge waves, allowing them to survive your hail of fire to get in close and tear you to pieces. Simply running and gunning might seem like a good approach, but it won't keep enough of the enemies off you, so a different tactic is needed.
Equip a rocket launcher with a large blast radius (the Goliath or one of the later model Stingrays will work beautifully for this) and a sentry gun of some type. Lay down the sentry guns and bombard the enemies with the rocket launcher as they come in. As soon as the ants get in range, activate the guns. This will highly slow the opponents' advance as the front line is constantly hit by the sentries. This will allow you time to pick off the ones that break through your wall of fire.
Make sure to reload your sentry guns before the first group is defeated, because a second group will spawn behind your starting location. Keep a cool head and use the same strategy on all three waves to prevail against this numerically superior foe.
Mission 27
You are stuck in the middle of a group of tunnels with enemies coming at you from all sides. You have three allies, all of whom have nothing but close ranged flamethrowers. What do you do? Grab an assault rifle and some sentry guns and go to work! Start the stage by dropping the guns as fast as you can, activating them as they deploy, and then run down one of the tunnels. This will make all the enemies that are not in that particular corridor have to come through the center area to get to you.
The sentry guns will be doing most of the work for you as you take down the group in the tunnel ahead of you. This will give you some time to find a place to hunker down in one of the corridors. If you can find a location with only two entrances that has a nearby bend (to reduce some enemy fire), you can remain there and take on incoming enemies while reloading your sentry guns.
Once reloaded, you have the choice of either breaking through into the center and repeating the process, or using the sentry guns for extra firepower to take out the remaining enemies.
Mission 32
This mission pits you and a group of snipers against a massive influx of spiders. This is made more difficult by the fact that the battle takes place in the middle of a ruined city, and the rubble makes it incredibly difficult to get a shot on the encroaching horde before it reaches you. The answer to this is surprisingly simple. Grab a close combat weapon like a shotgun or assault rifle for any close up work, and an MLRA.
Since the rubble blocks most of your shots, simply aim upwards with the MLRA and sweep from side to side to lock on to different spiders. This will create an aerial bombardment against the spiders. They can do nothing to defend themselves against this technique. It won't net you many power ups, since most of your attackers will die in the distance, but it will make dispatching your foes much easier.
Mission 33
Another mission against the giant quadruped fortress and it’s still not time to take it out, despite what HQ wants! Make sure you have a sniper rifle or rocket launcher to take out the UFO carriers in this mission, and back it up with whatever weapon you like to take out the ants and Gunships that will be attacking during the onslaught. A good shotgun will get the job done. To start, completely ignore your troops. They are ready to rush into combat and die a valorous death. Let the A.I. do that.
You instead, need to move to the left of the starting point until you are nearly at the western edge of the stage. From here, take down any of the enemies that are coming in and attack the first of the carriers. Attacking the carrier will make any enemies it has deployed come in at you, but you won't have to deal with the enemies that are under the other carrier.
At some point, the fortress will start firing its cannons and moving. Make sure you stay far away from it, moving north or south as needed to keep your distance. As long as you do so, the lasers will never come close enough to be any real threat, leaving you to simply take out the couple of UFO carriers and their attendant enemies. Let the fortress go its own way. Don't worry; you will have time enough to take it down next mission!
Mission 9
This mission puts you up against a new type of enemy, the Walking Arms or Hectors, as they are often called. These powerful mobile weapon platforms can be outfitted with a variety of oversized attachments on each arm to adapt to their enemies. The Walking Arms will start out at a decent distance from the shore, but will be constantly moving in and raining down destruction on the group of troops gathered there.
To make matters worse, a huge group of gunships will need to be taken care of as well. The thing to note about Walking Arms is that their height makes them very unstable and rapid firing weapons can easily throw off their aim. As a result, closing the distance with an assault rifle, preferably an RA model, is fairly easy. This will also cut down on incoming damage by a huge amount while allowing your supporting troops to dish out a large amount of punishment.
As for the gunships, there are two viable methods for taking them out while focusing on the more dangerous Walking Arms. The first is to use a missile launcher with a fast missile (MLRAs or Emeralds work well) and fire off a couple shots at the swarm of Gunships while focusing on the Walking Arms. This will let you take them out every time you get a spare moment and cut down on the amount of enemies firing upon your troops.
The other method is to use groups of sentry guns, preferably fast firing ones such as the ZERA. This will let you concentrate on the Walking Arms while the guns take on the Gunships.