First thing you wanna do is to shot the Regenrators Left Leg in order for him to fall. Best way to Shot off his leg is to use a shotgun but sniper can also work.

Now once the Regenerator is down to the floor you wanna bait his jump attack which is him jumping towards you. To easily dodge this just slightly walk towards him then walk backwards. Make sure you have slight distance before you bait his attack

Now that you successfully avoided his jump attack you want to stand right above him/slightly infront of him and start spam knifing him. When you spam attack him with the knife you stun lock the enemy and he wont be able to recover and stand up. So you just keep stabbing until it dies.