weak DPS doesnāt matter since the party will have a lot of additional damage (Blessing, Enchant Lightning, Enchant Fire scrolls, Aukuras:Goddess of Fire) all adds a ton of damage to the multihit skills.
We also have to consider the 24/7 boost of Lightning attacks[Lightning Hands,Zaibas], melee attacks[Hovering/Sonic Strike,Pheasant] and Zaibas damage via Divine Stigma.
What we see in the previous videos is how the runs are won by using autoattacks with multihits to bring Belcoper down.
Krivis also offers a boost/solution for this, providing Dainos C3 attribute, which adds up to 170 Aspd for autoattacks (great in combination with Chronomancers Quicken), so once all skills are used you can just autoattack the living daylight out of Belcoper.
Clerics have the great benefit of Safety Zone, a higher HP pool and Heal in comparison to Archers that also need to stack crit rate to be efficient, as Greyhiem already stated above.
You donāt really need to debuff him if you can just kill him with additional damage thatās not affected by defence.
Enchant Lightning is a great example, it offers a decent boost that doubles during Storm Calling [as it counts as lightning elemental damage, it deals more damage to ice element e.g.], with Enchant Fire and Sacrament providing additional lines that pick up this boost+the boost of Aukuras:Goddess of Fire.
It wouldnāt surprise me if this setup could actually pull off 10k autoattacks per line.
So while the initial damage isnāt so high,the portion that ignores defence is really high.