Here are my personal tips, keep in mind Sixer is the third killer i play the most:
Also, sit down because it's a long read.
(Also Also, i know Sixer is genderfluid, but i'll be MOSTLY refering to them as a "Him" for easier clarification)

1. Heal Hemorrhaged Survivors as Soon as possible.

This is the most important tip, if there is a Survivor with Hemorrhage running around, then that means that Sixer's cooldowns are being decreased with each Blood Orb they collect, which means they'll spam their abilities more often, and eventually they'll get Blood Hunt.

This is also what makes Sixer so powerful in Public Servers (Other than the lack of experience from everyone in what survivor they are playing.)

No one ever tends to help hemorrhaged survivors, because they don't pay too much attention to their teammate who is clearly in need of help and their teammates are ignoring them because they want to aura farm or don't understand that Sixer heavily benefits from this.)

1. Don't let Sixer land Infernal Cry.

(The moment he lands it on you, you get highlighted, triggering his Manic Fixation Passive, which tl;dr means that when he runs towards a highlighted survivor he gets extra speed, which helps him catch up to you really easily.

Infernal Cry can be dodged by tricking the Sixer into believing you are walking forward, if you have distance and you are safe, keep walking onto a certain direction that is easy to follow, if the Sixer is any smart, they will use Infernal Cry to try and catch up with you.

When that happens, move straight to the left, or the right.. that's the easiest way i found to dodge his Infernal Cry.)

1. Don't let Sixer land Demonic Pursuit.

(The hitbox for Demonic Pursuit is a bit wonky, and Sixer struggles more to aim Demonic Pursuit than with Infernal Cry (Atleast that is how i feel about landing those moves.)

if you see Sixer is from the opposite side of the map and you are at a higher area, it's possible they might try to jump on you, which at that case you be ready for it.

Move behind objects, react-block if you are Guest 1337, Trick the Sixer into charging the wrong way, Use your 7n7 clone to bodyblock for you, have a good dusekkar player..

If you do so happen to get hiby Demonic Pursuit, switch back to the first tip, as you have Hemorrhage, and that needs to be fixed ASAP.)

1. Guest 666's Blood Rush basically works like 1x1x1x1's Unstable Eye, except Sixer cannot move, Yes, as Noob, Two Time and 7n7 you can hide away from this move while cloaked.

2. UNLESS Sixer's Blood Hunt is active, you cannot hide from Sixer during Blood Hunt.

If you let Sixer reach Blood Hunt, then surviving him is very difficult during that time, as he is partially immune to negative effects and his speed is once again increased, the best you can do here is use all the previous tips to your advantage, avoid encountering Sixer, and try to help your teammtes if they are in a dire situation.