I'm still looking for low-effort way to beat deity. So far my only answers are "AP cheese" and "culture hoping not to get squished" which actually isn't as hard as it sounds with diplo abuse (I've won culture games with Ragnar as an immediate neighbor on deity).
I guess you could piggyback wars too.
The other excessively cheap way I've discovered that will occasionally work is to capitulate someone (say, early treb gambit), gift all cities back, gift them all of your metals, and SPAM THE CRAP OUT OF WARRIORS ---> gift them to vassal. Deity AI with their gobs of bonuses, 50% discount rate, etc can easily handle upgrading masses and masses of warriors, allowing them to over-stack even beyond other deity AI. That's when they start taking cities and snowballing. Oddly, if you don't make a demand or mis-treat them they never seem to break free

. Eventually, AI will kill everyone you DoW and you will "win conquest"................................