30k to 50k power gap, no ninja will save you from the power gap, that is he truth mate. Lets say I have a basic blitz team , Iruka, WB asuma, earthmain, Susano Sasuke(assuming I hoarded my seals for this ninja) lets say I have 90k power and I stacked my position one, all my runes and mags and refined and charmed equips is on my Susano sasuke, the rest are just supports. Then there you are you have 60k power (as you said above that -30k-50k power) with strategic ninjas with a balanced power build, no stacked ninja. The outcome is pretty legit, you will get nuked, Susano Sasuke will hit any unit infront of your team, you get repulsed dmg and the mystrery dmg that is a crit.

Going all-in for power is not enough as you said and that is true but you are a F2p you got limited resources, let the seal scrolls do the job for getting ninjas and the rest for power in that way you wont get left behind.