People Using Shotguns

Just remember to always roll.
Also you must know how to blind fire perfectly, it's not hard to learn and get used to with a shot gun.
Perfect reloads help too.
So first get to a moderate distance with the player who is calling you out to a shot gun fight.
Blind fire his lower chest or hips. After every shot do a roll to the side, and keep going.
After a couple of shots mix it up and roll to the other side and if you haven't killed him already, roll while doing an active reload.

People Who are in Cover and Using Lancers

Just take out your shotgun and take cover.
Move up a couple of spots and take cover (there is bound to be some type of cover), then keep moving up till you are pretty close.
He will have no choice and should be an easy kill.

People Using Boom Shots

This is pretty easy but may take some time learning.
The boom shot is a pretty concentrated weapon, it's not like a grenade that explodes a distance, it fires in a pretty small space but dominates anything in that spot.
So you have to think fast and have your lancer or shot gun out, if he fires think fast and roll to the side.
He will take some time reloading and thats when you roadie run as close as you can.
If he fires another shot just roll agian and watch yourself.
If he doesnt, get as close as possible to him and then you can just chainsaw him if you don't have the lancer out.
Watch out if they try and melee though.

People Using the Sniper Rifle

This is easy once you know what to do.
If you are being watched with a sniper just rush him, don't worry about getting shot because there is no way he can headshot you, and the odds of getting downed while doing this are very high.
So rush him ,he will take some time reloading and when he is about to shoot, roll faward and then you can either chainsaw him, or simply shotgun him in the head.

People Using Grenades

Watch out because cover won't always save you from this one.
If you are hiding behind something from people who have grenades don't bother taking cover, they will need to be killed quickly.
So take out your pistol for this one and also be very careful for tags.
So while you have your pistol, start blind firing him.
If you aren't good at blindfiring, just follow the trail of pistol shots and start drilling into him with that pistol, it's a 50% chance he will roll infront of you and tag you, or start freaking out and throw his grenade.
For the first 50% chance this will give you a great advantage to kill him, but you must be quick.
If he comes close to you, melee him with the pistol before he can tag you.
If he freaks out and throws his grenade, well just be quick and shoot him down to the ground.
Hint: If he does succesfully tag you, you will be dead but you will have a chance to follow him around and he will die with his own greande. Don't bother trying to roadie run after him, just simply roll forward and try to be as close as possible to him. If you have already got some hits on him before he tagged you, then it will be easier for him to die by his own grenade.

People Using Torque Bow

This is an easy task, first rush him while he is charging the bow up.
Then just take out your shotgun or pistol, and rush him while shooting, he should be getting low on health, and if he succesfully shoots another one, remember that rolling is everything.
If you are skilled at rolling to the side, you will evade the shot.
Then you can take him out with a couple of shots with the pistol or shotgun.
If you have been stuck with a torque bow shot, just come up as close as you can to the enemy (if you were rushing him). If u have been stuck while on the move, well... you're dead.

People Using Hammer of Dawn

He will of course aim at the ground and activate the satelites beam.
Hopefully he will miss and then you rush him with a pistol.
Don't aim, just rush him with a pistol and shoot.
If you are far far away from him, it will be risky, but remember to always roll to the side.
Keep rushing him and always active reload your weapon, that will give you any advantage.
Hopefully by the time you get really close he will be downed, or if you haven't killed him yet just melee.