people here said everything i guess.
Analye enemy-champ before u want to counter him.
-in early squishy/medium health
-snowballs like a boss
- is an assassin so if the q hits u, u wont get away that easy.
so now try to counter it:
1. Defensive playstyle:
If u wanna play defensive u need very high lane sustain, though its not easy to dodge the Q of diana. Vlad with his great dodge spell , or galio cause he's fkn tanky, should be a good choice.
2. Offensive playstyle:
If u wanna go agressive vs diana u need a champ that can snowball better or snowballs the same and has more sustain. it's getting a bit difficult here.
u could play Katarina (harass Q, and jump out of her Q with E) or maybe vlad (hes getting health back. diana isnt) . I personally favorise Akali. Why?
Because Akali's Q->hit comb is one of the best snowballs in early. and with W spell u can get invisible so she will often fail her Q and even if she hits u can get away. I use runes+ masteries to get + 10 ad so i have 8& spell vamp (akalis passive) by spamming Q i regenerate a lot of health and do lasthits. U surely need some skill to dodge and stuff, but since youre lvl5 your Q does so much dmg and gives you some health back. Diana cant spamm her Q in early, cause she will get oom quickly if she does.
Anyway Diana is a very strong champ and should be not underestimated. If she gets some kills or chances in early, she can truly get OP, like other Assassins.