Summon Board Passive selections:
Odin choose 4 / 6 / 7
Bahamut choose 3 / 4 / 5
Usuals:
Ifrit, Shiva, Ramuh, Brothers 4 / 5 / 7
Leviathan, Pandemonium, Diabolos, Alexander 4 / 5 / 6
For more details and help with summons, check Summon Hub!
A Spheres - mBRV spheres to help his overhead. ATK + mBRV sphere are ideal as he does need to do a few BRV hits to raise his Dark Matter.
Sephiroth is the easiest to get and the best boost (ATK & mBRV)
Firion, Zidane, Agrias are comfy 10% mBRV spheres.
Yang, Terra, Sabin, Shantotto, Ace & Twins are a little riskier but he can trigger these before his burst phase if any target is broken or you get a lucky crit.
Stacking sphere ("up to 5 times") effects aren't as great as he's dealing BRV hits about 4 times in a fight ideally.
Here's our Sphere Hub page to check more conditions.
Almost all of his HP attacks are based of Dark Matter. Dark Matter is his special effect (above his head) and stores the amount of BRV DMG he does (up to his mBRV). It is cut in half when Astos is broken. Open with LD and he will use FR and BT (order may vary) and how you decide to obliterate enemy HP is up to you. BT phase, share turns, Luna LDCA and delete in a few turns, the world is his plaything.
He still respects his HP DMG Limit (99,999 base) and can snapshot his FR's HP DMG Limit if he lacks BT (359,999 base w/ boards, just don't let him do HP attack mid launch). With BT effect active his damage based on Dark matter is uncapped (meaning he only respects the 999,999 hard limit).
This also means he does not benefit from mechanics looking for an HP DMG Limit like Quina. He also needs a lot of mBRV and HP DMG Up to reach the hard 999,999 limit because if he has low Dark Matter, it will be much harder.
You will also either rely on his own Force Time, spam echoes early and use his BT phase to kill stompable enemies (like you could do already) or you'd need an incredibly generic FR for Astos and allies to take advantage of.
Dark Thrust (S1) is 10 ST HP attacks based on Dark Matter (250%)
Dark Slash (S2) is 5 AoE HP attacks based on Dark Matter (250%)
Dark Warhammer (EX) is 9 ST HP attacks based on dark matter (300%) + 1 splash. Grants his EX buff: [Suffering of Fools]
Dark Glory (LD) is 10 AoE BRV hits followed by 7 AoE HPs based on Dark Matter (250%). Free skill use after. Grants his LD buff: [Dark Elves' King]
He wants his buffs for more mBRV and HP DMG up
Conflict Ultima (FR) effect provides HP DMG Up +50% to party during his force time but asks for:
Party member has special effect on turn: 40%
Magic attack used on turn: 30%
At a glance they seem nice, but a special effect is not something that can be given to a character. A special effect is an overhead icon (In Astos' case his Dark Matter special effect). Calls, BT effects and Spheres are NOT special effects.
This makes his FR tricky to hit for a lot of the roster and since Astos only uses BRV hits 4 times in a fight, he has a hard time hitting many other FR conditions.
Flare Star (BT) effect lets Dark Matter go beyond Astos' normal HP DMG Limit.
Party effects are mBRV, BRV DMG Limit, HP DMG, HP DMG Limit up.
For biggest funny number: FR -> S2 -> EX -> LD -> S1 -> S1 -> Finisher (Things usually die by the LD)
If you do rely on Astos' FR as the main Force Time, units like Aerith, Braska, Sherlotta, Shantotto, Terra, Yuna, Rinoa, Garnet, Deuce, CoD, Quina, Neon, Minwu, Raines, Iroha, Kuja, Yuri* and later Selh'teus, Leonora can hit both of his conditions. However since the main goal is to boost Astos' damage, as long as they have echo and at least 1 of the 2 conditions, they can just press the funny button and let Astos do his thing.
Ultima Weapon on him helps as it provides mBRV and HP DMG up - both of which he wants for funny numbers.
His damage stands apart from most units in the game in an optimal setting, however fights are not requiring this amount of damage. Astos should not deter pulling other units who provide actual utility or you will probably struggle against fights that have mechanics.
Reiterating again, Quina does not affect Astos while Astos has BT effect active.