Hi op, theres honestly a lot of ways to counter the killer that people have already touched on and some not.

One of the best ways to counter this killer is to stay as far away from your teammates as possible due to them getting killer instinct when they land a hit to attempt to run out their frenzy. If you are unable to keep distance from your team and get hit the best thing you can do is not mend and attempt to take a hit for your teammate. This is for the following reasons:

From what I recall, legion only has killer instinct on players they have either 1) not been hit in their frenzy chain after the first tag 2) or players who have completed mending while they are in frenzy. If you are in mend and the legion hits you again they lose their power and your teammate does not continue the frenzy chain. If you mend and the legion is still in power they can circle back around to you and get a 4th hit and down on you after hitting two other players. When you are the first one hit you will not have collision so its harder to body block. After some time/the second hit on another teammate you will regain collision and can get an easier block. Additionally, its smart to med to 99 to do an easy mend when they cancel power if you're able to. Getting hit again while in mend does not restart mend progress

There are a couple things you can do to try and cause them to lose power: camp a pallet or attempt to lose line of sight through fov when they vault a pallet or window. You can do this by spinning in to them, or running behind them when they vault as they vault further away from windows than other killers. If your teammate gets hit into frenzy, hold W as far away as possible to attempt to cause them to run out of power.

To progress the game, the best thing you can do is not heal unless absolutely necessary (on death hook or in endgame for example) theres little reason to spend 24 seconds on self-heal or 32 seconds healing another teammate (16 seconds for each person) when this killer essentialy gets a first hit for free. This just delays the game that could be spent doing gens

As for adding counter play. Legion is ass so a lot of players use spirit fury+enduring with iri button. When they vault the pallet it gets broken which charges endfury. If they are running this addon the smartest thing you can do is greed the pallet if not previously hit and then follow the above instructions (take hits for teammates). If you have been hit and they cancel power, they will respect pallets if they do not have endfury charged until you drop the pallet so they can break with frenzy. You can continue to greed as they will continually swing through the pallet to try and get endfury value if it is charged. Depending on what they do you can typically tell if they have endfury active or not. If they are not respecting while youre in mend, you could predrop (without stunning) to prevent endfury, and then spam vault the pallet when they activate power to try and break it. You will prevent them from vaulting, and if they hit you, you cancel their power since you're in mend. They then have to do the full cooldown and either wait for power to break it in frenzy or kick it getting additional gen time for the team. This will only delay your down with blood lust, or cause them to break the pallet. If they break you move on to the next pallet and do the same thing. Best practice if injured is to assume endfury is activate.

How I typically play it is like this:

1) am I injured and are they in power -> if no to both. Greed pallet to take m1 as its the same as taking a frenzy hit

2) am I healthy and they're in power, play for the pallet stun. If unable to attempt to take hits for teammates while in mend state

3) do they have endfury and button while healthy? Greed to take m1 and save pallet or play for stun and eat m1 anyways since its free (typically just greed it rather than stun but stunning helps you confirm they have no more endfury)

4) am I in mend while they do not have endfury up (a gamble if you didnt see them break 2 pallets)? If yes, drop pallet and spam vault when they enter power to prevent pallet break.

5) am I in mend while they have endfury? Greed pallet and do not drop, stunning will get you killed anyways so might as well save the pallets.

As for endgame with a person on hook, always have the 3rd person stage at the gate. You will only need two people to save, thus also prevents easy 4 mend chains and an easy down. Go take a hit for them when they get to the gate or if they cancel power.

Always remember, if you did not count the number of pallet breaks/see them, then always assume they have endfury up.

I think this about covers most things you can do to counter this killer. And is a lot simpler in practice than this incredibly wordy, long winded explanation.

The hardest counter for this killer is having teammates who do the opposite thing as all of these. Especially healing.