It's not f2p friendly, quite p2w actually, just not Ronin level p2w, but I used Edo Deidara, Kakuzu EGF, 5KM, earth main.

The gist of both 370 and 380 is you need a ninja who can do 2 things: 1. Stack up a lot of dmg during wave 1; and 2. Deals dmg to as many targets as possible with his/her mystery, so as soon as wave 2 hits, you can oneshot as many enemies as possible. Edo Deidara (with at least scaling passive sb) fits the bill.

There are similar ninjas to Edo Deidara that are more f2p friendly, but it's also harder for them to survive. You could look up a Youtube channel by the name of "Sirld kiri" for other ideas, I did look at some of those videos for reference while doing my NE. Though China had many more sb ninjas b4 they had exam 341-380, so not all of the options shown there is available to us yet.