Achievements
Complete each achievement to get the allotted gamerscore:
All Stages Cleared (Easy) (50) - Clear all stages on easy difficulty.
All Stages Cleared (Hard) (150) - Clear all stages on hard difficulty.
All Stages Cleared (Hardest) (200) - Clear all stages on hardest difficulty.
All Stages Cleared (Inferno) (300) - Clear all stages on inferno difficulty.
All Stages Cleared (Normal) (100) - Clear all stages on normal difficulty.
All Weapons Aquired (200) - Aquire all weapons.
Unlock Bonus Weapons
Beat every mission on a certain difficulty to unlock the corresponding weapon:
Genocide Gun - Beat all missions on Inferno.
PX50 Bound Shot - Beat all missions on Hardest.
Y20 Impulse - Beat all missions on Hard.
ZE Launcher - Beat all missions on Normal.
Zero Laser Rifle - Beat all missions on Easy.
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 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.