Finishing Halo:Reach on Legendary Solo is not impossible but it's not a cake walk either.  It's more about patience and conservation than anything else.  Following are some basic tips for pulling it off.

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• Let your fellow marines bear a lot of the weight.  When you have another member of Noble Team with you, they cannot die.  For that reason, move forward enough to get them into the thick of things and lay back.  They can thin out a crowd of Covenant before you have to start picking your shots.  But be aware of your surroundings.  If a Noble Team member gets knocked off a ledge or knocked into a spot from which they can't return, they will be no help.  an example is fighting the two Hunters on Pillar of Autumn.  If your team member gets knocked over the edge, he'll just stand far below and be able to do nothing until you reach another checkpoint.
• If you have marines that aren't Noble Team, don't count on them too heavily.  They can hold their own, but if you rush in with three of them, you might find yourself alone in a hurry.  Lay back an let them draw fire while you pick off enemies, just don't expect them to take too much punishment.
• Precision weapons are a good choice to keep with you.  The DMR and pistol are good as well as the sniper rifle and the combine rifle.  Don't count on the needler very much since the enemies will dodge before you can fill'em up.  Of course a rocket launcher is always nice.  The grenade lobber is really a POS since it's difficult to aim precisely and bounces when it hits.
• When facing Elites, don't forget the n00b combo.  Before you can take them down with a DMR or sniper, they'll take cover and recharge.  But if you hit them with the plasma pistol to drop their shields then switch to the DMR, you can eliminate them quickly without taking too much damage.
• Besides picking up a few extra credits, there's no real reward for clearing out a level.  So run, run, run as much as you can.  There's a lot of cover throughout the game so use it and keep moving.  Several levels can be navigated quickly by just running like crazy while using the available cover.  You will die, but you can make two or three checkpoints before you do.  Wash, rinse and repeat as often as possible.  There will be times when you have to hunker down and fight, but at least you don't have to do it every inch of the way.
• Grenades are great for close quarters, but aren't very effective is open areas.  On Legendary, the enemies will jump and dive out of the way, so don't waste grenades in the open.  And don't count on them to take out Wraiths either.  You'll need a bushel to do so.
• When dealing with wraiths, take out the gunner first.  The gunner is a dead eye and will light you up before you can even get close enough to jack the wraith.  Lay back and take him out before approaching.  Once he's down, you can run close enough to the wraith to jack it.  Be warned; when you get close, then wraith will dash at you so be ready to jump on.
• Watch for grunts with fuel rod cannons.  Besides the fact the cannon will blow you up, it's a good weapon to pick up once you've splattered the user.  Forget about rushing the user because they will shoot no matter how close you are.  Pick them off and then grab their cannon.  It helps with dealing with larger, more armored enemies.
• Swords are fun but not very useful.  Only a few close quarter areas exist where you can effectively use the sword.  Most of the time you will eat a ton of plasma before you can sword lock an enemy.  Don't rush a group on covenant expecting to get a sword spree.  You will more likely get torn a new sword hole.

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