Rough damage formula (Updated: wordings and some facts)
(Attack * Factor - Defense * Factor) / Factor * Power of move * any other multiplier such as crits

The base power of a move is definitely a flat multiplier according to experiment, where Confusion (21 BP) on Mewtwo did a little more than 1/6 of Psychic (118 BP), where Aerial Ace (45 BP) doing almost exactly half (100 BP) on Delibird which corresponds correctly after taking into account damage range (0.9x to 1.0x). This means that sync move (BP) level ups always result in a 5% flat damage increase regardless of everything (Stats, base power, enemy stats, etc.). To put it simply, a 100-power move is expected to deal double the damage of a 50-power move on any given unit.
Stats are a much more complicated matter, where double the attack doesn’t necessarily means double the damage (it does less than that under normal co-op circumstances, and more than that in levels where enemies are very weak, such as early story). My formula suggests that the factor resulting in the final damage is **the unit’s offensive stat relative to the enemy’s defensive stat**. This is backed up by how Screech from Persian was able to let a Lycanroc sync move (1/5) break past the 99999 damage limit after crits and weakness factor. This indicates that **damage exponentially increases when the offensive stat relative to the defensive stat reaches a certain threshold and vice versa, where damage exponentially decreases**. (Stats are never low or high enough in any decent co-op for this to happen as they mostly expanded HP pool to make bosses bulky instead of increasing their defenses, but low level enemies do close to zero damage to you, while bringing a low level unit to a EX results in receiving damage close to the 5-digits proofing the validity of the formula) **This means that overall, offensive stats of a unit has less impact on damage output than BP of moves under normal co-op circumstances, which is shown to be correct based on actual damage output of units** (Kris does more damage than Olivia with a crit despite having almost 150 lower Attack and only 67 BP higher in power). This also indicates that stats carry diminishing returns (The difference in damage between 200 vs 270 attack is higher than 340 vs 410 attack for example). As such, it is probably a good idea to focus on move power upgrades instead of Atk/SpAtk in the sync grid (if you can). The formula is also shown to be consistent with the main series’s damage formula at rough.