Flood SporeFlood Spores are tiny creatures that look a bit like a jellyfish. They attack by attaching themselves to their target. If they succeed, they target becomes infected and turns into a Warrior. They do very, very little damage, but sometimes travel in packs numbering in the dozens. When they are killed, they explode, damaging nearby creatures.
When encountered in groups of a dozen or less, fighting Spores is blindingly easy. All you have to do is stand there. They attack, and, upon hitting your shields, die. Your shields can easily absorb 20 or so Spores before they run out. If you encounter a few Spores, just take the hits and wait a few seconds for your shields to recharge.
When encountered in groups, the best strategy is just to fire a quick burst from whatever weapon you have at the moment, killing most of the Spores, and then simply absorbing the hits from the others with your shields.
No weapon is more or less effective against Spores, as any hit from any weapon will kill them every time. Grenades tend to be overkill, as do rockets, but anything else works just fine.
Flood WarriorFlood Warriors are Human or Covenant troops who have lost a fight to Spores and have become infected. They bear only a vague resemblance to their former selves, but can use any weapon found in the hands of a Human of Covenant soldier throughout the course of the game. They do not use vehicles (or stationary guns). Occasionally a wounded Warrior will 'play dead,' dropping to the ground and staying immobile until a likely target strays near.
The two best weapons to use against Warriors are the shotgun and the pistol. A shotgun blast from close range will kill any Warrior in one shot. Pistols do serious damage to Warriors, and two or three shots to the body will drop them. Additionally, pistol shots will sever a Warrior's limbs, and a shot to the head is almost always instantly fatal. The sniper rifle is equally effective, but Warriors tend to travel in packs, and sniper ammo is rare. Plasma weapons are particularly ineffective against Flood troops, and this includes plasma grenades. Frag grenades are as effective against Warriors as they are against anything else.
Flood CarrierFlood Carriers are bloated, walking Spore nurseries. They don't use weapons and have no regular attack, but if they get close to an opponent, they will explode violently enough to drain your shields almost completely. Furthermore, once they explode they release a half-dozen Spores. Carriers are at their most dangerous when they surround you, as they can close in on you from behind while you are busy killing the ones in front.
Carriers are easy to fight, at least as long as you know that they are there. A few shots from a pistol or assault rifle, or a single shot from a sniper rifle or shotgun will kill them easily, causing them to explode and release their Spores. Other weapons are less effective against them, but they move very, very slowly, giving you plenty of time to shoot.
Wise players won't shoot Carriers on sight. They are not a threat unless they are close, and they often travel in the company of Spores and Warriors. Watch the Carriers and wait until they are close to another enemy or two before shooting them. The resulting explosion will damage or kill all of the nearby creatures. Be careful; exploding carriers will also cause any grenades lying on the ground (such as those dropped by killed creatures) to explode on the spot, compounding the damage.
SentinelSentinels are offensive/defensive drones built by the race that created Halo. They appear as flying machines, armed with a dangerous laser that puts out a sustained, damaging beam.
Sentinels are extremely vulnerable to plasma weapons. A charged shot from a plasma pistol will take them down in one shot, and a few bursts from a plasma rifle or a needler are very effective. Shotguns work at extremely close range, and assault rifles will take them down in a pinch.
The trick to fighting Sentinels is to stay out of their way. Their weapons are only dangerous if you take a sustained hit. You can easily step out into their line of fire and take the hits long enough to fire off a shot of your own, then step behind cover to recharge your shields and get ready for the next shot. In the open, keep moving. Sentinels don't aim all that quickly, and a moving target can often stay completely out of their beams.