The par score for Floodgate is 25,000. To get a nice multiplier of 3.00x, you'll have to beat the level in under 15 minutes. It's a short level and can be run within that time even if you fight every battle. Put your setting to competitive score under options before starting the game so that you can keep an eye on your score and time while playing.
Heroic difficulty with a few skulls on will give you multiplier you need to achieve 25,000.
Getting kills without dying builds up your killstreak and earns you more points. Multikills also give you slightly more points.
This entire level has only flood enemies, who are not too threatening even on heroic.
Catch - enemies throw more grenades. Doesn't affect the flood.
Tough Luck - enemies will dodge more.
Fog - No motion sensor. You can probably get away with not using this skull if you find the lack of motion sensor annoying against flood.
Iron - Dying restarts the level. If you die, save and quit immediately, then resume from the last checkpoint to keep score intact. Doing this will reset your killstreak, however, meaning you'll get less points per kill until you build it back up again! It seems a recent patch causes the save and quit method to invalidate the achievement.
Dual plasma rifles are your friends and they're everywhere. SMGs aren't too bad, either. Try to hit flood where the tentacles come out of their chest, it's their critical point.
Grab the gravity hammer on the dead chieftain at the beginning and use it against the first few swarms of flood.
Don't kill any marines unless they fully transform into flood. You'll lose points otherwise.
Don't bother with the flamethrower unless you're very, very careful.
The energy sword is excellent for the pure flood. It's a one-hit-kill on the flood tank forms. Avoid getting swarmed or you will die. Bubble shield and Regen fields can help take out groups when you have a sword.
Stick to cover when fighting pure flood ranged forms because they're very precise and will knock out your shields quick.
You can use the turret on the level to take out some of the flood pure forms quickly.
WATCH THE FULL END CUTSCENE. Skipping ending cutscenes on any level when going for par score may cause the achievements to not unlock.
I was able to get about 32,000 with 3 minutes to spare. You've got a nice bit of room for error.
Unfortunately, the par score achievements in this game be can stubborn about unlocking. Personally, I had to play through the final level several times for the achievement to unlock. You may want to wait for a patch if you don't feel like testing your luck.