So.......... Awaiting the start of Season 6 I've been playing very sporadically but still having a lot of fun with Diana, even after 400 games with her in S5 alone! Definitely going to grind with her to Plat again (and maybe higher?!?!) in Season 6. Last 2 seasons began January 10th & 21st so I'm expecting S6 to start mid-late January as well. Only a couple weeks away! I'm really pumped for the new champion select, too.
I feel like Diana is even more OP now than in S5. Thunderlords adds another ~300 damage to her burst making 1-shots so much easier. It's kinda ridiculous how often I get "fed". Right now I'm on a 7 game win streak with her, and my worst losing streak was a 3 game slide 2 weeks ago. Even with that 3 game slide, I'm currently 12-6 in the past 2 weeks with her which is a 67% win rate.
Everyday I'm surprised Diana still doesn't see bans, increased pick rate, or even nerfs on the PBE. There was a Riot post recently talking about Diana seeing a rework in the future, which makes me nervous, but hopefully that's still far off.
I seem to have no problem getting her even as last pick. There was some Diana pro play last season and I totally expect even more this season considering she's only stronger. When her new skin comes out her pick rate will surely rise. I look forward to finally having a new skin (she only has 1 non-release skin atm) but I don't look forward to having to fight to get her.
Anyway, I've been wanting to do a montage of 1-shots across many games, but I've been slacking. Instead here's a highlight video of my latest game last night where I got into Plat 4 Promos for the 3rd time. Hopefully I can succeed this time
(Obviously not every kill is a 1 shot, but close enough)
If you want to counter Diana the best options are long-range mages with a combination of good poke,damage,cc, and sustain like Swain/Azir/Viktor or bruisers like Riven/Lee Sin/Irelia/Renekton/Darius/etc.
Oh, and my offer to Duo is still available, especially since pre-season doesn't mean much. Also with the new champion select coming it might be nice to have a duo/trio/etc, though the games could be harder since you're guaranteed to be against a duo+ also. Still should be fun.
Edit: Succeeded promos
Bonus clip from a Promo game