Without any surprise, the last boss of Diablo 4 Vessel of Hatred is the Harbinger of Hatred, Mephisto. Truthfully, the last fight might easily be easy or super hard, depending highly on everyone's builds. It gets really hard to fight when other players are forcing an endgame build that relies on a few rare items essential in the campaign, and you don't have those. With a build that features low single target damage, it's the same-you just have to sit in front of Mephisto for some more minutes, but if you know all the mechanics, then you will definitely manage to defeat him. You can defeat Diablo 4's Harbinger of Hatred, Mephisto, with this strategy guide.
Now, with the surprising treachery of Eru, Mephisto has been released from that stone-in other words, he was suspicious in that he never explained why he saved you from the river in the first place. In order to save Nahantu from Mephisto's demise, Eru went and fetched the stone and the vessel that would assist in resurrecting Mephisto.
Mephisto sends the monster Harbinger of Hatred to delay you and Neyrelle, so you two cannot stop Eru. As this is Mephisto's flesh avatar, essentially it is him, The Harbinger of Hatred.
At the beginning of the fight, the Harbinger of Hatred grants you a very limited space to work and corrupts the arena's floor with the corrupting touch. When Akarat gets well, he will join you in the battle, but until then, you are going to have to make do without his assistance. After he stands up, Akarat will wipe the floor clear of corruption and remove the Harbinger of Hatred's immunity. Till then, you can actually harm the Harbinger of Hatred.
In the next phase, however, Harbinger of Hatred will regain his damage immunity. The only way to remove this Harbinger of Hatred's immunity is via Akarat's power. In each phase, in the beginning, Akarat will change into a ball of light that you need to pick up to access his power. Similar to conduit shrine, once you absorb Akarat's power, your skills will transform into a special attack capable of breaking Mephisto's immunity.
You may seize the opportunity to inflict some damage to the Harbinger of Hatred when he is in a state to receive damage, until he reacquires his immunity, of course.
In spite of the fact that the Harbinger of Hatred appears to be corrupting at all times, as shown in the second image above, there is somewhat of a recurring theme within his attacks, carrying a constant sense, much like the fight against Lilith. He will then smash everything, slapping the ground on either of his sides with his ears and shoot and break into three projectiles; he will do a slam on both of its ears and launch from its skull five projectiles in the last slam.
This would be the animation for his assault, which begins once he starts to flail his body. Getting farther away from the boss and dodging its projectiles as they approach is one of the best tactics to avoid it. If you have access to the defensive abilities, you can completely ignore these mechanics.
Swipes, even against you being close to him, are a disastrous attack; each of those will take more than 50% of your health out unless you have enough maximum life, armor, or defense through passives or abilities-where you'd need to spam potions all the time, just to keep yourself alive. Since what you want to be doing is dealing consistent damage to the boss, you should save your defensive abilities for this scenario.
Of course, during the whole fight, there will be pop-up hollows everywhere, and you will be fighting them. Akarat will be able to temporally draw the attention of the Harbinger of Hatred, thus enabling you to take out the horde or deal damage to the boss in turn.
Mephisto, the Harbinger of Hatred, can be defeated provided you are able to dodge many of his deadly strikes. With Akarat on our side, this struggle became much more plausible rather than trying to face Urivar alone. The possibility of recruiting Raheir, a mercenary known for enhancement in armor, will give you a fighting chance if you've been having lots of trouble staying alive.
This is all you need to know about How to Beat Mephisto, Harbinger of Hatred in Diablo 4 Vessel of Hatred, use this guide to achieve it with ease.