General

i stopped at the ring, no point in really getting the rest

I did all 4 quests. The items arent that great. You'll need 120/60/200/100 btw; for medal/ring/emblem/scroll respectively. Not worth, imo.

Ghost Ship 6 is also pretty good, small map, 2 platforms, just jump and Sanrenzan everything. Better when the map's been Kishin'd.

Dim PQ isn't a bad choice for farming them. The mobs do drop them, and the rapid-fire spawning actually lets you feel at least like you're getting somewhere with the farming (with 2x drop coupons of any kind, you'll get at least 1 Ki for each wave that appears).

@rfighter: The ring is pretty good, (the hayato one is 5att and gives 5% boss damage, which is okay) the rest of it is outcalssed by tons of readily available items.

I've gotten the medal and the ring so far and am 30 sengoku ki's on the way to the badge... Let's just say it's taken me many many hours of farming with a mutant pig to get this many. For reference, I've gone from Lv.181 to 188 50% on the amount of solo grinding that its taken me.

It's absolutely atrocious and the payout isnt even really worth it.

EDIT: Though I will say, if you're high enough level MP3 seems like a good spot to farm them due to the ridiculous spawn there. I did all of my farming at SH and Kritias though.

Is it even worth it?

It drops, but drop rate is very low. Its drop rate is similar to the last two part of Gollux prequest. You actually need 380 Ki if you want to get everything excluding the scrolls. I killed ~1,000 monsters at Temple of Time with my Kanna (Lv 151) and only got ~15 Ki from the drop.

Get yourself a Big Spider familiar to help with the farming.

I got 120 of them already. The drop rate's just really low.
Also they disappear really fast, so pick them up as soon as you see it.