For ZWJ, my guess would be:
LV60: Leave scheduled valley (mastered 9Yang)
LV68: After learning Qiankun Da Nuo Yi
LV75: After learning Taiji Sword/Fist
LV77: End Novel
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In addition, I'm going to add in a special group for "formations/synergies", since these are almost always brought up and compared against:
Xiao Long Nu (LV45) + Yang Guo (LV42) = LV75 [玉女素心剑法 Jade Maiden Swordplay]
7 LOCH Quanzhen Disciples (LV35-41) = LV70 [天罡北斗阵 Big Dipper Formation]
Murong Fu (LV60) + You Tan Zhi (LV60) = LV82 [Extreme Synergy]
3 Du Monks (LV62 – LV65) = LV80 [金刚伏魔圈: Telepathic and Buddhism Power]
And maybe...
7 Wudang Heroes (LV38-LV46) = LV90 [真武七截阵] - Speculation. 64*LV40?! Maybe it's enough to take down Xu Zhu?
Which one gives the greatest boost?