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After breaking his barrier, you need to do enough damage/use enough skills with knockdown(no holds) to cause him to take a second knee. You then need to throw 2 spears into HIS right upper … (more)arm guard(the left upper if viewing Glas from player perspective). The spears, I believe, have to be from 2 separate people(have not tried with 2 spears from one person). (less)
Eochaid and Regina seem to open up with centered strikes on the party.
For Regina, walk forward so you're near the point that triggers the cutscene. For Eochaid, fall down from the starting ledge.
Try not to move as the fight begins, as this will trigger this black screen and give the enemy a head start to their attacks.
With eochaid, he will start with purely rushing you down (3 hit combo), firing his little causa buddies, or walk towards you, which the first was the one that ended up incapacitating you.
The first part of this is wrong. As said by Atherion and by me, fall down from the initial ledge.