I always Succor before pull at "Demon Wall" boss.

Succor make me feel like I'm playing easy mode of Demon Wall boss.
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I think he didn't mean "Double Buff" but double amount of HP of the shield.
btw
Yes, it is the same buff but different amount of shield.
Aldo 300 healing potency = 300 potency shield
Succor 150 potency = 150 potency shield
(actually the HP of the shield will be the same as the amount healed (also count overhealing)
And both skills will replace the previous shield. (am I right? not 100% sure)