i find it ridiculous that the NCR destroyed high tech group that uses power armor and energy weapons and yet the NCR struggles very hard to kill slavers with knifes and sticks. I mean if the Enclave were in trouble they can call down an orbital strike and blow its enemy to smithereens at least thats what they did against optimus prime. bah i doesn't make any sense :brokencomputer:

It was a long time ago, when NCR troops were like Legionarries how loyal they were to Tandi, there were less of them, so they were better equipped but they still outnumbered the Enclave at Navarro, and Navarro was very lightly guarded.

Because at that time, Bethesda didn created the East Coast Enclave, AND Adams Air Force, so, there isnt a Killsat

And the ones in California was weakened by the Chosen One first,

Then,

The Enclave decayed until the NCR neutralized them from that area

How so, the chosen one Nuked the oil rig thus cutting off there sole support. Navarro was only a Vertibird refueling base you know. that had like what, a dozen or so APA suits. the rest where non-coms and tech staff. Hell the Chosen One could wipe out the base himself if you wanted to in FO2. Why couldn't NCR do it after FO2? the NCR did have advanced tech too you know. IT could product combat armor and heavy weapons and guns.

Also Navarro isn't like a fortified bunker you know, it just a fenced in helo pad with some hangers and some comm equipment. With a power plant and computer core under ground.

Well, it is possible for these following reasons:

(1) Unlike the Mojave Wasteland in which it is far way from the NCR's home territory, the Enclave outpost at Navarro is a lot closer to NCR territory. Therefore their supply lines will be more secured and reinforcements can be deployed quickly. THe Enclave however, is not able to do so.
(2) President Eden has already ordered most of the surviving Enclave personnel to be relocated to the DC area. All they have stationed at Navarro was a small garrison. Plus they most of the Enclave were killed off at the oil rig to begin with, so there couldn't be that much soldiers left stationed there to start with.
(3) The Enclave's president Richardson was killed at the oil rig and all they have left of their leadership was Eden who is on the other side of the country. THe NCR on the other hand, was still under the leadership of President Tandi, the most popular and strong that the republic has ever seen

So to summarize, the NCR had the Enclave out numbered, outgunned and had better leadership. The reason why they were having problems with the Legion was because they were just as numerous as the republic, a thing that the Enclave lacks.

The NCR never actually battled the Enclave, or if they did it was not mentioned in FONV. The Enclave were weak in that area and thats why you only find a couple in the Mojave, but they're not gone completely.

The NCR wipe out and take over Navarro after FO2.

Well then reaper, there is your answer.
With Navarro wiped out the Enclave grew weak.
As for your other question why NCR can't destroy Legion but can destroy Enclave so easily...?
I wouldn't say "easily"... Caesar's Legion consists of 86 tribes and slaves. The Legion isn't just a bunch of slavers with sticks... They have tactics, Weapons (not just melee), armor, and ranks just like any other army. I think it's perfectly reasonable NCR is having a hard time fighting the Legion.