I like to use a rather stripped down loadout ... call it my CE-inspired loadout ... to make it challenging and allow for the picking up of other weapons on the maps to be a more viable and tactical option than just relying on that loadout.
-Jetpack or Thrusters
But if you're going for a super tough solo/LASO/Legendary approach, you probably want something with more teeth.
What I would do is slap together a couple or a few loadouts based on whether your expecting a lot of Covenant, Promethean, or vehicle heavy missions and switch between them as needed.
You may want to consider a DMR and a Plasma Pistol for those perky Elites. Actually, Plasma Pistols can be easily acquired from dead Grunts, so maybe a DMR and/or a Light Rifle so you can keep your distance when necessary.
Everything gets a little bullet spongy the higher the difficulty, so always try to pop crawlers and grunts with headshots. Watchers are just a few hits from a rifle a well. What I'm saying is, I usually avoid automatics on Legendary unless I have to use them.
For perks, perhaps "Shielding" and ... well, I'd probably go with "Ammo" again.
As for the Armor Ability ... I'd say Jetpack would be nice for getting out of a pickle. Regeneration Field would be real nice for staying in those pickles and toughing it out a little longer. Autosentry can be decent distractions and can help out, but at higher difficulty they're easily dispatched. I don't think the rest would be that useful against high difficulty AI. They see right through that Active Camouflage.
And I'd go with Frag Grenades. The other two types you can collect from opponents more easily than you will come across frags.
But that's just how I would approach it. Not everyone plays the same.