Halo 4 has (at least before they nerfed the other games) the easiest par scores to achieve, by far. Other than telling you to look at the individual guides, here's what I have to say. This isn't a complete level by level guide it's just a general one.
Firstly, here are my scores and times on Legendary
DAWN - 36073 - 00:17:13
REQUIEM - 27453 - 00:31:48
FORERUNNER - 29839 - 00:27:34
INFINITY - 112546 - 00:42:26
RECLAIMER - 45911 - 00:54:22
SHUTDOWN - 52399 - 00:41:28
COMPOSER - 83312 - 00:32:33
MIDNIGHT - 63888 - 00:34:53
As you can see, you ridiculously overshoot many of the scores playing on Legendary.
Requiem and Forerunner are a bit harder, so I'll focus on them.
Going off my above scores, however, I would recommend doing Legendary for Dawn (no skulls), and Heroic for Infinity, Reclaimer, Shutdown, and Midnight (no skulls). Composer I was able to do on Normal. I didn't get a good enough time on Reclaimer to have gotten par on Heroic, but I took a really long time, so Heroic should help you cut out that time.
For Requiem, I found this video (Credit to Nak3d Eli) which, while I don't want you to really follow it since you won't score that high, I want you to watch it for where the weapons are and things of that nature (namely how he points out the carbines and plasma pistols several times.)
You pretty much want to play on Legendary and kill everyone. Plasma Pistol + Precision weapon are your friend. So its a good idea to follow the video for areas where he actually fights the people, but don't skip enemies when he does.
Some other tips for the level -
Don't use the Banshee too long. Use it to take out the enemy banshees then get back down. The enemies with plasma pistols are annoying as hell.
Don't use the Ghost. Again with the plasma pistols.
For Forerunner, I was able to earn 25k in 28 minutes playing on Heroic. The new par score is 22k. You might want to put 1 skull on. I died plenty of times so its not like it was a clean run.
I fought every enemy up until I got the first Ghost. I skipped the enemies in these areas. Use the Ghost to kill the Elite running for a Banshee after the tunnel. Use the Banshee to kill the 2 enemy Banshees and all the enemies in the area.
When you go through the portal to the 'core', kill the first group but then run past everyone. Go through the right door and stick to the left. Most enemies should ignore you. I had to walk backwards with my hardlight shield near the end because of the gunner on the phantom.
When in the Ghost Gauntlet at the end, splatter as many grunts as you can; this can easily net you several thousand points.