You can go from an LFR raider to a top 100 world mythic raider in a single tier assuming you start the process at the beginning of the tier. Beginning of expansions are the best place to start this process. Many players are stuck in the LFR/casual raiding scene and can't seem to break that barrier into mythic content. Here is a straightforward 100% way to break that barrier in BFA.
1. Find a lower progressed guild at the start of BFA (World ranked 3000+) who will take pretty much anyone. Get gear. Get some logs.
2. Once you've acquired those, leave that guild and find a guild world ranked ~2000. Get some gear don't worry too much about good logs, they won't matter too much at this level of play. Also they will be more progressed which means more chances at loot.
3. Find a guild world ranked 1000-1500ish. This is where you want to focus and getting some really nice parses and don't die.
4. Find a guild world ranked 850ish. This is where logs start coming into play as guilds around this rank will pay a little more attention to your logs. Again, get better gear, get better logs.
5. Find a 300-500 world rank guild. Once you're in, you will want to focus 100% on getting some strong percentile parses and getting as much loot as you can.
From here on out it's simple rinse and repeat until you're comfortable in the spot you're in.
Extra tips:
*If you fuck up a lot and your warcraftlogs is looking like shit and/or your guild changing pattern is looking suspicious. Name change and start fresh. WCL and wowprogress won't track your new name.
*You pay money to play this game, no one else pays your subscription fee. Don't feel compelled to base your decision in the interest of people other than yourself.
*Logs are everything. When applying to a guild that is a world ranked 101< 99% chance they will take a loot at your parses and if they look pretty you will have a high chance of being accepted.
*Masterloot is GONE. That's right, you have the same chance as getting loot as everyone else in the group regardless of anything. Use this to your advantage when finding the perfect home for you.
A lot of people will say "this wouldn't work with my guild blah blah blah" but the sad fact is most guilds 101< don't care who the recruits are as long as they are decent players with logs to back it up! Especially with master loot gone, if a strong player is looking to join a guild, why wouldn't they accept them?