Yeh, do not buy cloth/thread, almost all f2p on my server are maxed level 27 and rank 3 armor and don't even do SA anymore, waiting for medium thread to be added (It should come after we get Minato and Raikage SAs, unless they change stuff around).

Other tips that i would do if i had to redo account:

1. Do not rank up, wait till there is event for battle armor and when you rank up during it you get 20% of thread towards next level, $$$$.

2. Once you reach level needed to do next difficulty look up a guide and only do that one, never do lower difficulties. Even hard is easy, once you beat it, you can beat it again almost on auto pilot.

3. After you reached level 27/rank 3, stop. Not worth using stamina here, you literally gain nothing, wait until they add medium thread to SA.

4. In general power per coupons for battle armor is rather low, so do not use any on it, just get it by playing. As f2p save all your coupons and only use them during good events.

The current Lucky Dice event is once of best ones, it has 2-3 return rate for coupons you put in.