Just based on what I've seen from my own playing/observations.
Match score is based on how much damage you do, multiplied by kill assists. ELO going up and down is very similar to match score (but not identical)
Supposedly there is some fluff for Spotting, AMS, and UAV's (etc) but it's not significant compared to simply raw damage and how many assists. The fluff (or lack there of) can move you up or down a maybe a couple slots at most. Which is definitely enough to effect your overall score if you are on the edge of being able to make it to T1, like me....

In other words, it's hard work to get to T1. As it should be. You have to do everything correct, basically. Every match you drop you need to be trying to get near the top damage for your team, so even if you loose, you still go up.
Number of kills doesn't seem to matter much either.
So you can either try and get ~350dmg and ~8 assists in a light (doable) or just pump out a 600dmg game in your favorite assault.
If your TEAM loses you better have a higher damage than
all your team, and/or almost the highest damage but more fluff.
I've had loosing games where I was #3 match score (on my loosing team) and went up, and games where I was #2 and went down. That's why it pays to carry airstrikes and UAV's and spot enemies by reflex. If you see a red blip, spot it.
Unfortunately, backstabbing and taking out IS XL side torsos, and head shots... don't really help your match score. Very likely if you went around and headshotted 6 enemies and your team still lost, your ELO would still go down.
So, if you REALLY want to help "cheat" your way up a tier (or two), go with high damage mechs. Brawlers. Something like a Boiler with 2lbx20, 3mpl, 6smr6, 300xl, 3jj, tc1, cap. Then hang out with the group and wait to fire until you get in optimal range. Play like a coward, don't share your armor until it's go time. Or a Purifier with 8spl (or 7 and ecm) and just orbit your assaults the whole match (don't scout, you'll die).
Either way eventually the scrum hits and you will shine.
with that Boiler build you literally only need to connect with 6 alphas to anchor a ELO up. If you can figure out
how to twist and how to connect in the same spot you can destroy any assault mech you run into (1 vs 1) in 3 alphas. If you don't want to shut down- you have to pause 1 second after your guns reset to fire again to get 3 alphas out of it, so take two seconds to twist left, then 2 seconds to twist back and one second to target the same component again or the next mech after one shoting the piranha.
Ah.. that's called tactics. The real trick is to get yourself that close before firing a shot or taking fire. Sometimes you die fast if red group pushes a corner on you, but if you can even stay alive for less than one smoking hot glorious minute of contact you will anchor an ELO up.
Next thing that REALLY helps get your ELO up... is communicating with your team. You don't even have to have a GOOD plan, any plan will work...if you talk it can encourage other people to talk. This is good. Cause it's a TEAM game.
You just have to play, on average, better than your own team to go up.
If you average 51% you will make it to T1.
Eventually.