The present progressive would be "the relentless attacks are breaking his spirit." What you have there in the first example is a cumulative phrase with a participle ("breaking.") The second example is a straight-up present tense in an additive sentence.
I think in this case, the meaning is slightly different, and you must choose based on that.
In the first case, the breaking of the noob's spirit is happening as the posts are assaulting. The cumulative phrasing makes the timing simultaneous. We watch it happen, right there. You can see End put up the noose picture, the frantic making of alt accounts, MHD posting a devastating sketch, and a hopeless final "I didn't mean to insult anyone, and you are all so mean" post with tearstains on it, and it's all happening *right now*.
In the second case, the comma-and additive phrasing ever-so-slightly makes it a FIRST-THEN timing. The posts assault. And the attacks break his spirit. The wave of venom and vitriol spew. Thara unleashes a single "Lol." Mizal notes that this is the last time she'll attempt to help someone quite so stupid. And then, the spirit breaks. It could be in the privacy of the noob's classroom, or a bit later that night as he sobs himself to sleep. It's only a beat later, but there's a definite timing beat there.
Therefore, since the noob's spirit breaks ever-so-slightly sooner in example number one, that one is preferred.
