Both heimerdinger and zyra are very good duelists, and in my opinion very good tops too. I've constantly tried katarina top, but it's unfortunately very unreliable. Zyra top is pretty much like a heimerdinger top, problem is they are very subceptible to ganks.
I wouldn't call either duelist, more like lane bullies vs melees
The problem I see with zyra top over heimer is that she literally has to snowball as if she ever goes even into mid-game she'll usually get 2-shot by anyone smart enough to get within close range of her. While tankier builds could help remedy this (like liandrys+rylais+zhonyas+banshee's and potentially a zz-rot), heimer seems like a far safer option.
Try the On-Hit Veigar Build in Top. Rush Frozen Mallet first to get AA slows into your E. If your good at farming his passive, you don't need to buy AP on him, and on-hit builds work on everybody if you know how to fight.
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if a zyra met a renekton I would laugh so hard how easily renekton would kill her
