Highlighting this post that you notice. I am fairly sure you are interested about this:
4* Max level (60) Garnet using summon (ES Explo) _without_ any eq / abilities / buffs on, using max level (40) 2* Shiva.
Let me recap my findings about how Espers interact in battles to help you out.
-- Esper damage is based on the stats of the esper, not the summoner. AFAIK, there are no buffs nor any gear that can be equipped to boost esper damage currently on Global.
-- The only spell that indirectly affects esper damage is Imperil, which raises the damage of the esper by a flat 20%.
So, what's the big deal then? Where is there smoke that perhaps signals fire where no one is looking?
-- Esper damage appears to be a hybrid magic calculation based on MAG and SPR. Espers use the full four-digit MAG and SPR stats in the damage calculation.
-- Magic damage, as everyone knows, is based on the attacking stat(s) squared.
-- EVO MAG +% raises the base stats (not damage as the wiki says) of the attacking esper BEFORE being squared (thus, EVO MAG +30% is the esper equivalent of equipping a MAG +30% on a mage).
** Shiva's base MAG is 5500 --> 5500^2 = 3 025 000
** Shiva with EVO MAG +30% = (5500 * 1.3) ^2 = 5 112 250
5112250/3025000 = 69% damage increase expected. (Note: in my testing so far, there are other modifiers that appear to be in play, as the actual damage increase is in the 72-73% range)
** Max 5* Garnet achieves EVO MAG +40% total, yielding (5500 * 1.4) ^2 = 5 929 000, or an expected 96% damage increase (likely double with other modifiers) over non summoners.
So, right now, objectively, 5* Garnet will outdamage 4* Rydia with Shiva. With Imperil, maxed 5* Garnet + maxed 2* Shiva should be able to break the million damage barrier in ES.
In the long run, how can this be exploited? As you are also in PAD-GAF, you should know this analogy from that game. Think Scarlet System. Garnet and Rydia are the only units in the game AFAIK that have Haste in their LBs. Garnet is even more bonkers in that she gets a special passive ability at 5*s that gives her permanent 2x haste on her limit break on top of the haste provided by skilling up her LB.
**If** full system mode can be achieved (and there isn't an artificial block put in place by Gumi on Global), pouring out that kind of damage consistently and freely every turn would be practically game breaking.