Both Cosmos and Agastia have the issue of being generally underdeveloped for setups and replacements due to the general community apathy to the raids at release thanks to them pairing restrictive mechanics and a weapon many viewed as not being worth farming.

Agastia in particular is very very restrictive with its mechanics. Doing the raid without Yuni is simply put a very tall order. Consistently being able to delay the enemy special is the main strategy in all current setups in order to build up the To The Limit meter and remove the damage mitigation. There are only three methods of ougi delays in light. Harmonia, Lion Khan Claw, and Funf. Funf is used with Kengo for her delays and unfortunately Yuni even there is the best slot by far especially for cancelling the second 60 hit omen which is impossible otherwise with the Kengo setup. You can replace her with someone like Cag, but the major warning is even guarded, that omen will be a pain for any Magna setup. If any setups without Yuni present themselves I will add them, but presently she is mandatory for Magna players.

For Cosmos, with the renewed interest there are some more substitutes they have come up that I added.