Not sure what scope of the question is, but assuming you have teams of fully awakened units, leveled espers and skill tree...
where a surprising number of players remain undeveloped is not levelling their skills, upgrading their vision cards and setting up good sets, their equipment and getting good TMRs
TMRs are usually just slot fillers, have stats, or a support ability... a lot of brutals and difficult content, especially higher floor towers will increasingly rely on damage boosting skills, among many others mediena, miranda, rosa, cecil TMR giving you 30-50% more damage 3 turns.
If you mean the ratings in duel and guild war, those are just sums of stats and are less important than strategy
Until they invest in vision cards they will remain at a plateau, regardless of racking up 20-30 fully awakened MR/UR
to sum it up, +5 equipment gives 13 or 15% more damage, good properly built espers give 15-25% more damage, elemental party VC gives up to 35% more damage, atk/mag 15-30%, damage type as in slashing or missile etc, again 15-30% more damage... and there are even elemental eater cards that give a further 20% damage