You will need a par score of 25,000 on the Storm to get the achievement. The par time for the level is 15:00. For this method, you are not likely to get the 3.00x time multiplier, but you should still be able to get a lower multiplier.
It is possible to get 25k on normal, but you'll need skulls and you'll need to be very close to par time. I played through again and actually found it a bit easier to reach this score on Heroic, with a few easy skulls on. You'll want to aim for killing as many enemies as you can and beating the level in less than 25 minutes.
Getting kills without dying builds up your killstreak and earns you more points. Multikills also give you slightly more points.
Put your settings on competitive score to keep track of score and time during play.
Famine - Weapons that enemies drop have less ammo.
Fog - No motion tracker.
Iron - Dying restarts the level. If you die, save and quit. Then resume and your points will be intact. Doing this will reset your killstreak, however, meaning you'll get less points per kill until you build it back up again!
Tough Luck - Enemies dodge more often. Brutes are prone to berserking.
Be sure during the level to destroy enemy vehicles and not just their occupants. You'll get extra points. Also, it helps to know the level. There's a couple areas that are very large and have power weapons scattered about to assist with taking care of vehicles.
Before opening the first door for the marines, kill everything. As soon as you open the second door, hop into the back of the warthog and mow down all the enemies.
Destroy one of the ghosts with a plasma grenade you may have picked up and hop into the other one. Stay on the upper level of the next area. Run over or shoot the Brutes and grunts up top. There will be marines running out of a building that has enemies being deployed on it. Kill these enemies, and grab the missile launcher on the roof. Watch out for banshees in the air; don't waste you missiles on them because they're good at dodging. Instead, use 4 missiles to take out the mortar wraith. Use the remaining missiles to kill the grunts in the ghosts. Commandeer one of them, and use it to destroy the other ghosts. Go near the 'ramp' that leads to the other wraith. Use it as cover to shoot the turret brute on the wraith. Kill any nearby grunts. Jump the ramp with the ghost, board, and punch the wraith until it explodes.
The door to the next area opens. You can hop back in your ghost to help take out everything. Watch out for power drains. There's a turret in the next room that you can use to take out the drones quickly. Be sure to grab a BR and a rocket launcher in this room.
Hop in the driver seat of the mongoose. Hopefully your marine can take out the wraith tanks and possibly kill just the brute in the chopper. If that happens, take the chopper. If not, keep trying to get angles on enemy vehicles so your marine can hit them. Hop out and use your rocket launcher to destroy the vehicles. Try to keep a couple rockets for later in the level. You may get taken out in this area, so be prepared to save and quit. It may be possible to use the missile launchers on the hill to take out the phantom, but I didn't attempt it.
When Johnson starts his dialogue, it means the scarab is about to appear. Use the lift up near the missile launchers to get to the top of the crane. The scarab will walk underneath and stop. Jump down on it, kill the enemies, and destroy the scarab. The door up top will open. If there are any intact covenant vehicles still around, destroy them.
The next area has packs of brutes, and they're good fodder for grenades. When the hunters show up, use your rocket launcher on them. If you have any rockets left, take out the brutes behind them. Watch out for the chieftain.
Before getting to the AA cannon, there's a dead marine with a sniper. Pick off a couple enemies if you can, though you won't have much ammo due to famine. Proceed to kill all enemies around the cannon. Surviving marines may move up to help you, so beware of them taking your kills and robbing you of points. Use the brute's fuel rod cannon to take out the AA quickly or use sticky grenades.
By the end of the level, I had about 17,700 base score with a time of 23:30, netting me almost 36,000 score total. You got room to make mistakes and take your time.
WATCH THE ENDING CUTSCENE or the achievement may not pop.
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.