Cassiopeia w/ Black Propaganda, Cybele, Guan Yu, Oberon, Zephyrus.
The highest I've solo'd is somewhere in the 140's. I don't think that my union even gets above 150's/160's. Works out for me though, since I play manually, so I'd probably get exhausted if we went higher.
You guys know why Cybele/Oberon/Zephyrus are there. But why Guan Yu? That fifth slot kind of doesn't matter much. Caspiel would be better, and I do have her. But I kinda got tired of having to use her debuff every 3rd turn. I should swap her in for ults at least, though. Among the rest of what's available to me, I opted for Guan Yu having two direct damage skills for rage gauge manipulation in case I screw up somewhere. I usually don't need two, but mistakes have been made at times.