Might as well report here. I finally got around to tackling Belias yesterday. I managed to sub-30, but it is very RNG, even worse than Famfrit:
Rikku (Lm1+LM2): R4 Sapphire BUllet, R3 Exhausting Polka, Mako MIght (CSB/BSSB1), one water boost
OK (LMR+LM2): R4 Voltech, R3 Entrust, DMT (mUSB), one water boost
Elarra (LM1+LM2): R3 Protectga, R4 Curada, Ace Striker (USB)
Alphinaud (LM1+LM2): R3 Ogopogo, R4 Lunar LEviathan, Dualcast Summon RM (SSB), one water boost
Vivi (LM1+LM2): R4 Chain Waterga, R4 Chain Waterja, Gathering Storm (USB), one water boost
RW: Guardian
Magicites: Famfrit (+L8 Blade Ward/L10 HP Boon), Famfrit (+L15 Mag Boonx2), Geo (+L15 Empower Water/L15 Atk boon), Phoenix (+L8 Blade Ward/L8 Spell Ward), Evrae (+L10 FAst Act)
My first version of this team was with LD Rydia w/ BSSB2 instead of Alphinaud. It took a while to find a turn rotation that would even get me out of Phase 1 without someone dying, but eventually I got it and mastered in 33 seconds or so. I did this twice, then swapped Rydia for LD Alphinaud and Deployment Tactics. I thought for sure this latter team wasn't going to cut it, since the damage ramped up MUCH more slowly than with Rydia. I renewed Rikku chain around 15 seconds and I had barely done 15% damage. But somehow I got a sub-30 out of it more or less on my first try. That reflected damage really piles up near the end, I guess. It was very close, though, 29.49 or something like that.
I fought Belias 4 times with this team, got 2 sub-30s (both 29.XX), 2 in the 31-32 second range. I went back to the Rydia team for 2 more fights, still didn't get any time better than 32-33 seconds. So despite the slower ramp up and lack of imperils, the Alphinaud team seems to be slightly better, but they're both very RNG, dependent on a lot of dualcasts and Vivi triple casts. Very annoying and inconsistent. Good thing this isn't a major farm.
Oh, and I also tried this with Tidus CSB in place of Rikku, since I saw some sub-30 teams doing that. Quick cast from Tidus CSB allows you to do somethings early that you can't with Rikku CSB. But under Tidus chain, my damage was too low and it never really went anywhere, so I switched back to Rikku.