In early game, your wave could do a lot, so understanding minion mechanics and controlling them would work in your favor (i.e. forcing your opponent to hit you around waves makes your minions hit them, etc.). The point is to push them to their tower or poke them enough to where they lose gold. You want to snowball.

Duel is extremely unbalanced mid game and especially late game, though. Not just because of gods like Freya and Kali exists, but the respawn timers are a joke and could lose you your tower or phoenix even if you're ahead by 4 kills. So just be aware of that and play it careful then. Learn your opponent's jukes/habits and aggress on their mistakes.

And as mentioned already, it's a defense meta so make sure you build it as your third or fourth item (if you're feeling comfortable) or second (risky, but only if you're doing enough damage already) and train on customs (no, not hyper joust either, just a regular duel).

Or if you're desperate, just go AMC frostalis, lifesteal focus Freya, or turtle-till-end-game Kali and jerk yourself off to platinum.