Defeating them will temporarily keep them from attacking, though Vegnagun can revive them both if they're both KO'd, and they can revive each other if one is KO'd. This will, however, make them waste a turn, allowing for the player to unleash their attacks on Vegnagun if they wish. Multi-hit attacks such as Black Sky and the like are recommended, as they can hit the Redoubt and Vegnagun at the same time. Otherwise, having Reflect up for most of the battle will help in dealing with the Redoubts spells.
Lacrimosa is taken from the ending of the Dies Irae poem, which describes the day of judgment.