First off, yes, as you keep playing matchmaking will figure out your true skill and match you as evenly as possible.
Although I believe that the ranks are playlist dependent. If you play a lot of Infinity Slayer, and then go into Slayer Pro, it might not match you properly at first.
The DMR is a tough weapon to use, I'd recommend trying out the Battle Rifle, it is much friendlier
.
Make sure you're consistently checking your radar to see the positioning of enemies and use that to your advantage! Arrow above/below the dot means enemy is above/below you.
Positioning is a HUGE aspect of Halo.
Straight up 1v1 face offs are very difficult, try using positioning to ensure you do damage first!
Half of dodging grenades is learning when people use them. That comes from experience.
If you're really having a tough time with grenades there are several things you can do.
1) when you see a grenade indicator, immediately react! move the opposite direction and jump if necessary, the vertical height will help propel you as far from the damage radius as possible.
2) there is an armor mod that decreases explosive damage done to you, and increase your explosive damage... I forget what it is called... Maybe "Explosives" lol? Idk.
3) try thruster pack or hard light shield. I personally use thruster pack all the time, if I see a grenade and can't dodge it using the traditional methods, I'll thrust in the opposite direction and dodge it. Just try to be careful you don't thrust out out of cover and get shot. Make sure to strafe back into cover as soon as you're safe from the grenade!
Stick to power weapons you're comfortable with, don't experiment if you're having a bad game.
Sometimes it is just best to run. If you're losing a battle, you can take cover, let the enemy charge at you, and use their aggressiveness to your advantage with a well placed grenade or melee.
Really try to pick your battles.
The primaries all have their effective ranges.
AR/Storm Rifle/Suppresor are all really good at short range, and will beat precision rifles even if they land all their shots.
Battle Rifle/Carbine are really good at short-mid range.
DMR/Light Rifle are really good at mid-long range.
The DMR, carbine and light rifle are all skill weapons, and might be difficult to use effectively.
Focus in the gun fights, keep cool.
Often battles will come down to a single shot difference.
That is all I can think of now