Yes, Halo 4 on Solo Legendary is VERY easy. Here are some tips that should make it that way for those having problems.
Steps to completely annihilate your foes.
Always pick your weapons wisely. I NEVER use the assault rifle or any of the other weapons like it unless I am forced to. Weapons such as the battle rifle and the magnum can be your best friend. Always go for head shots.
Battle rifle(and similar weapons) can one hit kill:
crawlers
grunts
jackals
Knowing this you can understand that holding these type weapons can make the game much easier. First you take out the one hit kill units to open space and then you go for the bigger kills. ALWAYS take out the jackal snipers as one of the firsts. That will open a huge amount of room that they try to take away.
For covenant I will take out the one hits and then go for the elites in several different ways depending on the situation. Using as little ammo as possible can help. The Elites range in number of hits to kill depending on their rank and your accuracy.
For prometheans I will then take out the flyers because they have the ability to shield the knights. They are easy to kill with few shots as long as you do not let them fly away to recharge their shields. Once the flyers are all dead I can eliminate the knights with ease. Below is a list of weapons and how many shots it takes to kill a knight with them.
Abilites of knights:
-shields are about the same as a gold elite but recharge faster
-teleport to random locations or right behind you
-their backs are mostly invincible
You can kill Prometheans with very few shots:
Knights:
Battle Rifle(shots to the head) : shields down with 3 shots and dead after 2 more
Shotgun: shields down with 2 shots and dead after 1 more
Railgun: shields down with 1 shot and dead after 1 more
Sniper(shots to the head): shields down with 2 shots and dead after 1 more
Alien tech weapons can follow about this many but will be different amount depending on how the weapons are used and how accurate you are. Example: The scattershot will most likely take more shots than the shotgun because of its loss of accuracy at times.
(sorry if I am off on the numbers but that is about the number I remember for the shots.)
Remember that you do not have to kill everything at once and finding good cover helps. For ranged weapons you can use a tactic that confuses the stupid AIs. In most cases you can kill enemies even if they do not light up red. When this is true the AI will not pick up your character and will either do nothing but die or will simply shot and backoff and then repeat in an attempt to find the source. Using this you can kill most units before they do anything but you have to be careful about situations and placement or it will not work. Regardless of this trick, you can still kill them easily with ranged weapons in a comfortable range.
Use the environment to your advantage. You can use it to get good cover or to maneuver around the enemies and ambush them. You can also use it to stop the enemies from doing the same.
Closed ranged weapons(ex. used shotgun) are also very effective if you know how to use them. First you must understand how to approach the enemy without being detected or approaching quickly getting cover and then approaching again. Once close enough shoot the enemy at close range which will cause a flinch. Be careful to not shot the prometheans in the backs and to not give them a chance to retaliate. While they are flinching you can shot them again and again until they are dead which makes them easy kills.
The main idea behind the campaign to make a strategy for what you need to do in certain situations. Always take out the weak but annoying and then finish off the strong when they do not have the backup. Use your environment to your advantage. Understanding this you can beat the campaign easy on legendary solo. Even up the odd some will you.
Side Notes:
There are multiple missions that have areas that have vehicles and then no vehicles. There is a reason for this. The vehicles would break the other areas. On one of the first missions I used a warthog and a ghost to get another ghost up on a ledge that was no vehicles location. Not only could I zoom past enemies in part but I could also kill units with ease. Sword elites do not attack you when you are on a vehicle…Also for areas with vehicles. If you do not have an objective to kill all the units then do not do it. You can use the vehicle to zoom past to the objective instead.
As a final note, if I left anything out be sure to add it in the comments. Just be glad to help some of the newer players with the journey. 
Additions from fellow players:
-Blot shot can one hit kill crawlers also. Now you have a use for all those bolt shots just lying around.
-Plasma pistols are a quick way to get rid of shields. They hurt your shield so why not the knights or elites? Take out their shields and then finish them with a magnum, battle rifle, or another assorted weapons of the same likeness.
-Needles can pretty much kill anything fast in this game…apparently that includes knights too. (unproven for legendary(any testers for this?))
-Abuse your armor abilities. Last I checked they didnt have feelings so why should you not use them just like your weapons. They can be huge help and the protection services do not come for you if you abuse them so what is there to lose? Pick your abilities wisely so that it helps for the situation. I personally like the sentry the most for its uses. Need cover fire? It does that. Need to just take something out? It does that too. What more could you ask for?
*Additions from the players:
Duo Vandal
RagingNooob
CPSDIABLO
IAmLuke21393