Just realized Chain Challenge can be the new best way to farm SP (and to a lesser extent, EXP and HM). While like story chapters, they yield half SP and EXP after the 1st clear, surrendering at the last map apparently does not constitute a "clear". If you take a 10-battle Chain Challenge (which costs 30 stamina), beat the first 9 maps then back out, you retain the SP, EXP, etc. gain for the 9 maps you have beaten. When you repeat the process, since you haven't technically "beaten" this chain challenge, you still get 3 SP per kill instead of 1. And the 10-battle Chain Challenges at normal difficulty conveniently have the enemies at 4* around lv35, which is just perfect to grind for your 5* lv40 units as the enemies are low level enough to be auto-battled by your OP units while still above the threshold to yield SP.
We don't know when IS will (or even, if they will) patch this, but I'm leaving normal Chapters 1 & 2 Chain Challenge "unbeaten" for the time being (and maybe 9 & 10 too, for its reinforcements, but the trade-off is the higher difficulty). I won't go all in to grind SP just yet, but it is certainly nice to have something that can come in handy in the time of need. We are talking about 30 stamina for 9 auto-battle level of easy maps featuring weakish lv 35 enemies. It will be crazy with weekend SP bonus.