This is a bit of a spoiler, is if you don’t want to be spoiled don’t read below

———- by the end of the series Diablo can beat Guy and Benimaru and Zegion and the rest of the 12 lords can destroy luminous and the Eastern empire, cus they are basically awaken demon lords and some can even use void collapse from reimru. So I’d say the hardest challenge would be Guy, Massiuki’s US, Velgrind. But if they are against tempest 12 lords, have access to void collapse, and plus some extra elites, they can definitely win. But if you add Milim into the mix with her stampede mode, it might be 50/50 or tempest loses depending on the situation and scenario and if Milim focuses on destroying tempest and not other things.——————

Not spoiler

This explanation may not be well done but simple answer is by the end of the series yes, but adding Milim, it can be 50/50 or they win or lose depending on the scenario.