The nuke in your case called Taunt ))))))
You need a good defended stack with a lots of Arlekins.
They will cast Taunt and some enemies will attack your stack.
Anyway, it will be much faster, cause you have bonuses and rivals have penalties and not vice versa. Also, it will be automatic battle and I recommend you to turn on "fast battle regime" in options, or otherwise you should do some houseworks)))
All other advices are still useful. If you can damage your rivals with magic, you should do it.
I also could recommend, if you have access to Ice Mana (Letum Frigus), Ice III, Snowfall, it is the most dangerous spell.
But Arlekins with Taunt - is the key!
And it's a very Balseraphian solution)))
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Another way called economical blocade.
Such a huge army requires huge maintenance. So, you should isolate this city, capture other, destroy all improvements.
Also check on advisor screen how much money your rival has.
With a time all units should be disbanded.
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Another way - diplomatic.
If you conquer enough cities and win enough battle, you could vassalize your rival.