The Tormentor is a very slow boss, which of course doesn't mean you should underestimate him. Most importantly, dodge the attack he does with a sickle in his right hand.
Occasionally, he'll also try to spit a poisonous substance at you (the above screen), but because it dissolves over a large area it may not be enough to just dodge it. The best thing to do then is back away from the Tormentor as soon as you see he's getting ready to spit.
Hit the area around his right arm in which he's holding the sickle. Don't try to attack his left side because he likes to hit the character with a huge outgrowth. These attacks are unexpectedly fast and thus difficult to evade.
Naturally, I recommend using Death Grip to instantly pull yourself to him after each unsuccessful attack. You can also consider the Reaper Form, but I suggest using it only if the Tormentor is proving a bigger challenge than expected, because soon enough you'll face other bosses. Keep hitting him until you empty his health bar.