At around 60% HP, Heolstor enters phase two.
This phase begins with him flying into the air and slamming his sword on the ground, causing some sections of the ground to explode after a brief delay. You'll need to dodge or block the initial ground slam, and then dodge or block the explosions again, or otherwise run to a safe section.
His sword will damage players with a random element or ailment.
Heolstor's move set remains the same as in phase one, but his attacks make fissures appear on the ground, erupting after a delay. You need to keep track of the boss and the fissures, as getting hit by either can often result in death.
He can also perform a new combo attack, where he will crouch, then slash at a player, and then slash again after a brief delay. This attack can be blocked, but it is usually safest to dash through Heolstor and get behind him. You'll need to dash again through him when he faces you for the second set of slashes. After the combo, he will fly into the air and slam the sword into the ground, and erupt the ground directly beneath you. This final slam requires either a Greatshield, improved dodging, or invulnerability to block, as it can one-hit K.O. most Nightfarers.
While this phase might seem overwhelming at first, using the same techniques from past phases is a great strategy: Keep your shield up, or sprint and dodge preemptively until you recognize an attack pattern that you can punish. Keep your eyes both on Heolstor and on the ground to avoid fissures, and use your Ultimate's invulnerability period to avoid his strongest attacks.