You can get Seals from completing the Castellum, you don't even have to finish any of Calus' games.

Dude you can "dismantle" then get it back from Benedict. Or you can finish the quest using lfg, there's your two options.

[quote]FREE THE HEAVY SLOT!!! Allow us to dismantle or store the quest gun in vault to free up the slot. We can buy a new & higher level one from Benedict in the tower. I have a low level Legend of Acrius shotgun locked in Heavy weapon slot that I will never complete because it is impossible to assemble a willing raid team to complete the quest.[/quote] Use lfg