Hmm. Personally, I like solo play on Heroic and co-op play on Legendary (which I've had precious little of in Reach). I find that to be the sweet spot, as adding a second player more than cuts the difficulty of surviving in half. I'm hoping some Legendary FF makes its way into Matchmaking eventually.
My general approach - for both Score Attack and regular Firefight solo play - is to find locations with three features: 1) a vantage point over the battlefield so you can see where the enemy is coming from and what they are, 2) which also has a limited number of ways to access so you don't get surrounded too easily but 3) which also has more than one way out so you can escape when surrounded or out of ammo/health.
The way the maps are designed, most have areas near the main spawn rooms and ammo cratest that work well.
That's my approach to solo play. When playing with others, I roam a lot more over the battlefield (I tend to use Sprint almost exclusively) and flank the enemy squads. Most players tend to huddle up near the primary spawns and ammo racks. But by moving outside those areas I can romp through packs of guys pretty quickly as they're either isolated or distracted.
Two clips I've posted before I got a kick out of show that approach, where I'm letting the other players occupy the center and moving to the peripheral to take out the stragglers and then flank the rest.
One,
two.
Don't use Armor Lock.
And also,
dance like a mofo.