JKR can get you a win against literally every team. Some are more difficult than others (GG droids, DR/Malak), and require a bit of strategy modification, but there's a reason he was meta for so long.
You'll have a hard time against:
DR
GG (not nuke)
Padme
And even these are beatable - swap in Thrawn for GK against DR, and you can get a win with careful play. Use Bastila lead with JKR and you can beat GG with some patience. Careful play will win against Padme (minimize use of assists until Padme is dead).
JKR lead works fine against GG lead Separatists Droids too, I can still routinely take down g13 teams with r7 GG and B1 with everybody on my g12 Revan team (I usually use Hoda instead of GK when using them on offense) still standing.
Love tap magnaguard until b2 comes available by TM gain, focus fire down b2, mark and swat GG, clean up.
JKR lead works fine against GG lead Separatists Droids too, I can still routinely take down g13 teams with r7 GG and B1 with everybody on my g12 Revan team (I usually use Hoda instead of GK when using them on offense) still standing.
Love tap magnaguard until b2 comes available by TM gain, focus fire down b2, mark and swat GG, clean up.
It's the same strategy with Bastila lead, and you still need JKR to make it work, she just gives you more cushion by slowing down the crazy TM train that Sep droids can get going by limiting debuffs.
I haven't tried it with relics yet, but I can't imagine it'd be that different than the strategy at G12 (mark and fracture DR, bring him down with basics, hit him hard with GMY for the kill, clean up), with the caveat that you need your JKR team to be on approximately the same level of gearing (GMY in particular).
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Swgoh.gg
And even these are beatable - swap in Thrawn for GK against DR, and you can get a win with careful play. Use Bastila lead with JKR and you can beat GG with some patience. Careful play will win against Padme (minimize use of assists until Padme is dead).
Love tap magnaguard until b2 comes available by TM gain, focus fire down b2, mark and swat GG, clean up.
It's the same strategy with Bastila lead, and you still need JKR to make it work, she just gives you more cushion by slowing down the crazy TM train that Sep droids can get going by limiting debuffs.
Non malak Drevan would be harder, better to have Thrawn sub for GK
GG Nuke is not a problem, GG lead could have some problem
Padme for me as long as you mark her and kill her quick so she doesnt give prot up bonus from bonuses will work fine.
BH is not much a problem.
Swgoh.gg
I haven't tried it with relics yet, but I can't imagine it'd be that different than the strategy at G12 (mark and fracture DR, bring him down with basics, hit him hard with GMY for the kill, clean up), with the caveat that you need your JKR team to be on approximately the same level of gearing (GMY in particular).
alternatively, swap GK for Thrawn and fracture drevan
Use high relic Jedi team , and Thrawn to keep Darth Revan calm
I don't have an issue with Padme on offense assuming I am similarly geared. Just focus down Padme and everything is easy after that.