I'd go with something like:
Miku
- Enhance ATK Down
- Enhance LS
Wait until tomorrow so I can check how well Miku sparks with Ensa-Taya before setting the SP on this one, just in case they don't work well together.
Ensa-Taya
Either:
- Enhance LS spark damage
- Enhance BB ATK buff
- Enhance Spark Damage buff
or if you know you can get that one as a friend lead, instead use:
- Add/Enhance Spark BC buff
- 70% spark damage
If you use the second one, make sure it activates SBB before the one with the improved spark/BB ATK buffs.
Juno-Seto
- 2 turn mitigation
- Reduce BB cost
- Pick a buff your squad needs (Ailment Immune, Stat->DEF Convert, BC/HC drop rate or better HoT)
- If you picked a 40 pointer, get the DEF passive too
If you want to use Juno-Seto as colloseum leader, you may want to consider improving her LS instead of her BB cost. You could also improve her LS
and her BB cost, but that stops you from picking a second buff. Note that Miku has ailment immunity on LS, but isn't necessarily usable in all content (i.e. trials are generally immune to crit damage)
As for other units, well... that depends on what you have, and you'd probably want to build it around the buff options you chose for Juno-Seto.
Miku and Ensa should be used as leader pair for crittable content. I haven't run the exact numbers, but their output should come close to double Avant - Miku brings 350% crit damage/50% ATK, Ensa-Taya brings 250% spark damage/360% ATK. Double Avant would be 300% crit damage, 330% spark damage (assuming an equal buff in squad to Ensa-Taya) and 400% ATK - which looks bigger, but if Ensa-Taya is easily sparked the fact that she's HP-scaled should outweigh Avant. It can also be argued the crit damage is worth more as you're unlikely to sphere for it, while you're likely to sphere for spark damage.