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I play zyra and diana quite a bit on TT. I always build diana with 9/21/0 and tanky as I can go.
Diana : BFT, Abyssal sceptor, zhonyas(well its changed but same concept), frozen heart(because 100% chance your facing bruisers), and whatever you feel for your last item.
Zyra : sweeper, Wooglets, then ap items with HP/DEF depending on the team you are facing. Zyra's E,plants, and ult are enough to own as long as you stay alive.