1st questions.... how come when i put 2 clips in a guy with a jetpack he doesnt die and when he lands u puts a burst into me and i die when i have full health? um how come whenever anyone shoots me with a burst shot from any weapon i die instantly while i put a whole clip into sum1? umm how come when i throw not 1 but 2 g-nades literally next to sum1 they dont die? idk questions like that and dont say cuz im a noob nope its not that soo stick that answer up ur u kno wat... ok on to the problems that need to be fixed.. 1 make it to where if u put a clip into sum1 then smack them with the butt of ur weapon kill back into the game cuz having to hit sum1 twice is pointless and usually ends up with sum1 having the better connection winning umm another make the sprint longer or at least faster reload cuz common the spartan is a beast he could run farther than that and yea i think thats it...

Depends on what weapon you were using

The Guy with the J-Pack is high in the air, accuracy is a MUST to take him down, a DMR/Needler rifle/Pistol/Sniper rifle/Beam Laser Rifle would do the trick, If you were using a Assault Rifle, you have a 75% likeliness you're gonna miss
As for when he hit the ground, what was he doing, did he melee you twice? Did he use a Power Weapon?

Also alot of your questions may have been because of Lag, Reach has plenty of that, so it is likely that the grenade didn't get him due to lag...

Most of the time, A Melee battle ends with Both Spartans/Elites dying if done at the same time, if Spartan A hit Spartan B first, Spartan A can hit again before Spartan B can counteract Unless Spartan B Runs and shoots Spartan A, killing him, but Spartan A can also Shoot....ya

the jet packs tke too long to take off it should be faster and y is the handgun better than the AR