Elias was out of breath but he was holding on just so he could cast the spell. After suffering an injury, his mana system automatically activated and kept him alive long enough for Onyx to help.
'The Succubus' wings were laced with Dark Magic. I could not even move my body… if not for Sir Onyx, I would have died.' His heart and brain remained functioning even when he underwent shock and immediately rushed to support his dying body.
As a cleric he had little vitality compared to warriors, but two Arcane Ores were spent to almost immediately keep his brain and heart working. If the Succubus' feathers reached his heart and paralyzed it, even Arcane Ores would not be able to keep Elias alive.
He had expended a lot of mana and stamina all so he could retain his function. Keeping the barrier active until its designated cooldown was a job he had to do to keep the others from running away.