Bloaters are the final stage of Cordyceps infection. Joel and Ellie will have to face off against these monstrous and spore-grenade-tossing enemies several times in their journey. While you can unload tons of ammunition into the Bloater's face, using different weapons and different strategies can make short work of these hulking brutes.
Kill it with fire.
Fire is the easiest means of killing a Bloater. Tossing a molotov cocktail at a bloater and scoring a direct hit will catch it and any other infected around it on fire. The Bloater will flail about and scream, but it will survive up to three molotov in one life. Ideally, you'll be able to catch the bloater on fire with a molotov and then watch as it stumbles once more through the pool of flames you created. This will light the bloater on fire once again.
In Colorado, you'll find a flamethrower. This weapon is the most effective at killing bloaters as you can stand back and relight the big monsters as many times as you need. Don't spray a continuous flame on the bloaters as that's likely to deplete your gas canisters too quickly. Instead, light them once, watch them burn, and light again if they're still alive after the fires go out.
These flame weapons are also super effective against clickers, but it's smart to conserve at least one molotov and a little flame thrower fuel specifically for bloaters since they'll reappear in some pretty difficult situations.
Lead them away from water, so that they won't drink.
Very late in the game, you'll come across two bloaters in a very claustrophobic underground area. These two monsters will also be standing conveniently in waist high water. This is problematic for two reasons: it limits Joel's movement and it allows them to douse any flames you might have just ignited them with. The bloaters will be practically immune to your molotovs and flamethrower in this water, so what do you do?
Get them away from there! There's a passageway and a few concrete blocks standing out of the water on the right side of the tunnel. Getting the bloaters to come to you will allow you to use your flame weapons and funnel them into a specific area where you can ensure the waist-level water won't put the fires out.
Shotguns to the face.
If you don't have any other flame-damage weapons, it's time to whip out your trusty shotgun and blast these mother ****ers in the face. Plain and simple. Do it over and over again and you'll eventually bring the disgusting spore-lobbing freaks down.