Your best stacks?

[Shinz]

Im still confusing about my stack so im trying to do coliseum to get some good mirage

[it's a gabe]

The mirages you get from the coliseum (the ones that were DLC in the PS4 and Vita versions) have some good skills, especially early on. They're surely worth having.

Myself, I've been experimenting. I'm almost 20h in and I still don't feel like I have an optimal pair of stacks. I did learn, however, that using blank spaces can be really helpful to access stronger spells or provide specific statuses you want to augment further. It can also be helpful to pay attention to your stack's stability and keep mirages you think might be useful later on in your roster. Even if you don't have a mirage as part of a stack, it will receive experience as long as it isn't stored away.

[Fel]

Mostly depends on what you are facing to be honest.

Depending on the enemies, having a strong stability can be more important than pure raw stats, stacking to get specific elemental properties and attacks is often a good way to dish out powerful attacks while reducing what's dealt against you as much as possible...

I am no expert, but I found that having a powerful monster in the L spot can actually net you better results than having a good M if you invested enough points in your L monster (especially if it has 3-4 evolutions, so you can stack the boosts from multiple boards).

If you don't care about all of that, just look at the growth of the monsters from the book in the twin's room (or individually from the storage thing), it's a decent indication on what a monster can be good at, so that you either stack up strengths or cover weaknesses.

[Scarlet Crusade]

One of my favorite stacks from PS4 was Cactuar / Mist Dragon. They have supplemental automatic healing abilities, Haste, 1000 Needles, and in general just great raw power. I used it from when you can get it (around chapter 13-14 if I remember right) until the end of the game.

Other than that, you can't really go wrong with the elemental summon babies. They may not seem like much at first, but when they evolve into their mid forms is when they really start getting beefy, and near the endgame where you can have them go into their full forms, they're even more powerful than the star forms you get way earlier in the game. You can either pair them with themselves (like Bablizz / Shiva star) or if you'd rather use the mid form, you can pair them with a matching elemental (like Tama / Affrite) to just be really excellent at that element when you need to use that to hit enemy weaknesses. Just be aware that this is a high risk/reward setup, be wary of elemental weaknesses this kind of setup entails and don't use it where you'll get hurt.

[AnonTwo]

early thus far, i can only recommend two easy ways to stack. It's worked for me so far

1. Multiple characters who know "Fire", "Firaga", etc. Switch out whatever spell you're going for. Having 2 characters with this spell will unlock the next tier. So two Fira's gets you Firaga.

2. If you just want all around useful guys early on, make sure to use Sylph with someone if you're using them mostly for support, and Tama or an Elemental Baby for your main offense. Sylph basically has all the heals and Balance (Balance fixes stability so you don't fall).

Another reason to make sure you have Sylph early on is if you plan to overlevel, Sylph has access to tickle and subdue. Tickle is super low damage but knocks stacks over in 2-3 hits, and Subdue is basically the equiv of Pokemon's false swipe (non-lethal damage)

othhhher than that, keep in mind that some characters learn field moves, and you'll need them to get all the secret goodies

Stroll: Just hidden stuff, doesn't matter who does it
Sizzle (Ifrit baby has this) breaks ice
Smash (name?): I know the large evolution of the golem's has this. It breaks walls, opens up a hidden area in the Ifrit/Shiva/Ramuh area.
Flutter: Floating Eye and Halo Moogle in the coliseum have this. Gets you over gaps. Also needed to get into hidden area above

So basically:
-Stack characters with the same spell to get better spells
-Look out for characters with lots of utility moves to make things a lot easier
-Field moves are needed for most hidden areas and even progress.

Lastly, keep in mind you can scroll the stack setup panel until it shows you what skills you will unlock with combinations. Makes it easier to play around.

[PocketYoda]

So far still low but i'm using Floating Eye and Sylph and Copper Golem and Mandragor.

[(deleted)]

Personally I like L monsters like Behemoth with strong counter and sandworm which counter using spells (1/4 HP damage) when attacked. One allows me to tank and counter using attacks while healing. The latter allows you to disregard defence and inflict percentile damage to HP, also have spells that deal 2 element damage, constrict is AP effective and there is an attack that instant topple at the cost of toppling. Instant topple is invaluable in fighting high level coliseum with stacked enemies. I also like Behemonster in XL role, may as well use in L form too to grind weaker mirages since the boost in physical related stats is above average.
For M I do like the topaz carbuncle which has regen, def buffs and fancy glow moogle for S. The forest serenade is random targetting and heals more HP than curaga, that's 5AP vs 7. This is very all rounded and you will outlast everything while making minor adjustments to other party composition. I never bought a single curative item as the loot and setup seem to self sustain. Unlock -ga spells early as mirajewels as that will increase damage output to those stacks toppled by sandworm.
Other notable mentions are Ifrit, Shiva, Undead princess, kagura flan, goblin princess.
Only died twice in this game- once to the princess flan+ marlboro menance, the other to mindflayers but I believe these 2 requires specific preparation due to status ailments.
Early game search for valefor in coliseum, it's one of the better monster available early if you need very high topple.