Dialga in PvP

On Dialga i think, DB+ IH should be basic set
Take on mind, that machamp can have Rockslide as second move and hurt both of them. SB Mewtwo looks better , especially if have Ice Beam as second move to deal with Groudon. Non SB Mewtwo also should works, but need TB + IB combo. I would also try Lick/BS Snorlax as lead to break sheilds with optional second move EQ to deal vs Metagross or Ttar

Lugia, Giratina, and Dragonite are solid partners. The latter two share a Dragon typing with Dialga though, but they cover up Dialga's issues quite well. Giratina lacks a Ground immunity, but it has the best Metagross coverage out of the three, another Pokemon that can give Dialga trouble. Gyarados also works, but its fast move options aren't very forgiving.

One of those 3 + Dialga covers most things. You're left with a few things to cover - Opposing Dialga, Melmetal (somewhat), Metagross (if Giratina is not picked), and Groudon (if the Flying types are not picked). A Swampert or Groudon fits decently into these last slots. Mamoswine to an extent can work if Giratina is picked. Snorlax is also a fine pick in general. Note that these last two aren't particularly reliable in beating Dialga, though (especially Mamoswine).

Congrats on your perfect Dialga! I also got lucky and snagged a perfect one, who is now maxed out with DB/IH/DM. It's a lot of fun to use in PvP!

So first off, I wouldn't personally recommend Thunder on Dialga. While it might be beneficial in some circumstances (Lugia for instance; I don't see Gyarados that frequently and Kyogre even less frequently), Draco Meteor already does such absurd damage that you're better off going with that more often than not. If you find that your Dialga is often struggling to fire off DM, then consider swapping to Thunder and see how it works. Iron Head, however, is a must, imo. Mostly because it can be fired off quicker than Dialga's other moves (and does quite a bit of damage, even to neutral targets, thanks to Dialga's huge attack stat).

I would also recommend that you let Dialga take up one of the back slots on your team. Don't lead with it, and try to save a shield for it. So, as you probably already know, Dialga gets eaten alive by Groudon. It's not pretty. Unless, of course, you still have a shield and your opponent doesn't. A stream of DB, plus an unblocked IH, followed by another stream of DB is enough to take out Groudon if you can shield through the first Earthquake (it won't live to get off a second one). Machamp and, to a lesser extent, Metagross give Dialga the most problems so get ready to swap if you see those two (especially Machamp).

I usually partner Dialga with Lugia/Dragonite (who can easily take care of any fighting/ground issues) and then a Groudon of my own. Still testing the lineup for Dialga.

Have fun with the new hundo! May it serve you well!

PLEASE stop posting your opinions as facts it almost never the best choice against dragons and rarely even a good choice when they have double dragon moves....

Congratulations!!! You must be thrilled! I think it would be really good for ML as you can use it last once the shields are used up for 150 damage plus it doesnt take extra damage from dragon types! Groudon would be the only real problem i could think of. Congrats again!