IMO, Hallowed Ground is much more valuable in the Snake phase than anywhere else. It's the toughest phase in the entire fight. You can either use HG to circumvent the debuff stacks from the little snakes or to run out of the divebomb spot early and grab Snakes 4 & 5 without a healer. In any case, Hallowed Ground gives you a huge strategic advantage during this phase.

For Liquid Hells in the final phase, Hallowed Ground is definitely not needed. In fact, I MT this as a Warrior (No HG, obviously). All I need to do is simply rotate slowly around Twintania, keeping her in place, and making sure I'm never standing in more than one Liquid Hell. The party (particularly melee) might need to shift their positioning so they don't get hit by Plummet as I move.

Never move Twintania in the final phase. It's really beneficial to have Twin on top of a Neurolink for the entirety of the phase, with the OT (or melee DPS) occupying the Neurolink to absorb Hatches.