Compared to other events, DJMax is much less friendly to farm due to the length of its maps. While you can't fully auto-plan any of the maps aside from E1-1 (or E2-1 when retreat farming), it's still not too bad overall especially as we can skip the two most annoying maps and use medals to exchange for their limited drops instead.
With that said, here are some farming routes, refined from Ceia's video guide, linked below.
E1-1 Thunder Farming Route
As shown, killing an extra enemy before heading to the enemy HQ achieves S-Rank, granting a second drop chance.
* The video guide features a direct route for enemy HQ, which fails to get S-rank.
The above route thus roughly doubles efficiency.
E2-1 A-91 Farming Route
- Move Combat Team one node to the right.
- Deploy the Dummy. End your turn so the enemy walks into your Combat Team.
- Swap, don't forget to resupply, and auto-plan to enemy HQ.
* The video guide features a route that goes straight to enemy HQ and misses the S-rank.
The route described above grants S-rank for a second check, roughly doubling efficiency.
E2-1 SRS Farming Route
Autopath one node to the right, then to the node directly above the HQ. Deploy a dummy, swap, and retreat.
Note that although the relevant portion of the map is symmetrical, do not go left first. The dinergate will walk into you instead of the Nightmare with the SRS check, and you will only get two checks (and possibly get your combat team killed).
Using M1911 MODII for this map will require at least a 3* Fairy, but is at least twice as efficient. This route is also the best for farming Platinum Medals.
Whether or not you can farm this map with M1911 unscathed will determine whether it is more efficient to farm M1887 with medals from this map, or to farm E2-3 instead.
* The video guide deploys two combat echelons, which almost doubles your manpower consumption, as well as requiring more micromanagement.
The above route spends 12 manpower instead of 22, which seems trivial but will add up; 500 medals will take 167 runs, which translates into 1670 manpower saved - enough for three fairy crafts and then some.
Most importantly, resources aside it is more afk-friendly.
E2-3 M1887 Farming Route
Pt. 1 Step 4 can be autoed.
Pt. 2 Step 2-3 can be autoed.
Pt. 2 Step 5-6 can be autoed.
Don't forget to retreat Zas after she lures and kills the last target enemy in her area!
* The video guide features a route that involves fighting the boss with a 2AR1HG echelon, which incurs much more costs for deployment, supplies, and repairs. It involves 13 battles for 6 checks.
The above route achieves a comparable 5 checks for 11 battles, but for reduced costs, as described.