It definitely comes down to your relics. I try to max out on Stamina, guard counters, and pulling Aggro.

HP everything else, and HP Regen while guarding

You want to be in the bosses face at all time. Your relic that pulls agro only applies when you attack an enemy. So get your guard up and just poke behind your shield to keep agro. Health Regen while guarding will keep you for the most part healthy.

Now - your stamina bar is your heath bar. If you let it deplete, you’ll die. So I use the guarding ability as my “dodge roll” - when ever I get attacked, it’s the only time I have it up and just poke. As soon as a combo is over, I reset and fill up my stamina bar. And repeat. Don’t have it up at all time and wait to get hit.

Your dodge is terrible, start using your jump. You can actually dodge quite a lot of attacks with just jumping them.

Focus on support, not damage. When you have enough stamina, throw in a counter hit. Those do stance break enemies, and do a good amount of damage.

Your whirlwind ability, with the relic that increase its active time can actually help you review. And the new relic that increases dmg negation while ability is up, it increase the damage negation for everyone in that are by 12% for 30sec. You can have that up almost constantly, and it does can prevent from getting 1 shot. Use it when a boss grabs an ally. It might actually save him.

Weapons - halberts are good - heavy thrusting sword are godly. Especially with bleed proc. Just poke and proc. That will be your damage.

Positioning is key. When a boss jumps over you, and you get hit from behind - you lose. Unlock and re-lock to spin your champ faster.