Defense Down

Win 10 battles with any Rebel Admiral in your squad - Palpatine Journey Guide x 10, bring Admiral Raddus for the AOE.  I just did tier 1 since it's just win a battle.

Attempt to inflict Defense Down 100 times - JKL Journey Guide vs Wampa.  You'll get 5-9 per battle, or do Tier 3 JKCK with Baby Cal on basic.

Defeat 50 Imperial Troopers with Mon Mothma present - LS 6-J normal, 11-13 per battle. 

Defeat 100 enemies with Rebels - Tier IV CLS journey guide 17 per battle

Win 20 battles with a full squad of Droids - Palpatine Journey Guide. R2, C3PO, Chopper, K2SO, and Chewpio. 

九妹的第三技能可以群体减防御

Gungans I think.  Jabba/Boushh is possible on Chewie, but they didn't last long enough for me.  There are also a few inquisitors that give defense down.  La blue girl (the jedi vanguard one) also gives defense down for something jedi-like.  However, if you have Gungans vs Maul, second to last tier will get the job done.  There's probably quite a few more, Dash and many others give defense Down.

I did a cantina node so I can use both LS and DS characters. 

What node and who you use will depend on where you are in the game. You want to be able to win but not defeat enemies too quickly before you can inflict the debuff. 

For me, I did Cantina 8-G with 4-LOM (on basic), IG-90 (on basic), Ninth Sister (on 2nd special aoe), Lobot (on basic), Quiggold (on basic aoe). Not much damage, but lots of defense down. Quiggold was especially good for this. I got 62 defense downs in one battle. 

You can find more options here: https://swgoh.gg/characters/?ac=defense_down