Zekrom, along with its counterpart Reshiram, came out of the Gen 5 Movepool lottery in good shape. Being stuck with Dragon Breath hurts its Dragon-type performance compared to the plethora of Dragon Tail/Outrage Dragons; however, Zekrom received pretty much the best Electric-type moveset it could have hoped for in Charge Beam/Wild Charge. Backed by a massive Base 275 Attack stat, Zekrom is the undisputed #1 Electric-type, with significantly higher DPS than Electivire and bulk higher than Raikou.
On paper, the unboosted Zekrom duo is possible against a specific few movesets with maxed counters and extremely good RNG.
For the rest of the population, Windy Weather will be required for the duo.
For the unboosted Zekrom trio guide, click here.
Minimum viable levels assume Best Friends, no Weather, no Dodging.
Italics indicate a Legacy move.
If you're attempting this duo, consider switching to the next Palkia if your current Palkia is going to faint before using its next Draco Meteor.
The reason for this is to minimize energy loss - you want every Dragon Tail to contribute Energy to a Draco Meteor. If your Palkia wastes time using Dragon Tail but faints before it can use Draco Meteor, that's a lot of time lost and can make or break your run.
You will most likely need 12 maxed Palkias for this duo - the DPS check in this fight is not lenient enough for you to spend time Max Reviving. A second team of maxed Rayquazas may suffice for this duo, but Palkia is better.
Wild Charge is a fairly straightforward moveset to face. Garchomp doesn't faint enough in this matchup to necessitate a relobby, so the challenge is in fishing for good RNG. If you ever lose any Outrages to bad RNG (ie. dying to a Charge Move right before using Outrage), that run is most likely dead; this fight's DPS check is even stricter than the Flash Cannon duo.
Two teams of maxed Rayquaza (preferred) or Outrage Salamence and extremely good RNG are both required for this duo.
Charge Beam/Crunch is the easier moveset to go against, as Rayquaza only dies 22 times (11 times per trainer), which allows you to avoid a second relobby.
Dragon Breath/Crunch is harder, as Rayquaza dies an average of 24 times (12 times per trainer); without very good RNG, you will have to relobby a second time, which will most likely kill the run.
Minimum viable levels assume Best Friends, Windy Weather, no Dodging.
Italics indicate a Legacy move.
Duoing Zekrom in Wind is much, much easier than in neutral Weather, although it is still fairly demanding in terms of minimum requirements. Rayquaza is king; the more high-level Rayquazas you have, the better.
Flash Cannon isn't very different from Crunch in terms of overall difficulty. The only major notable point is that Flash Cannon is 1-Bar, which means it's easy to predict when it's coming out, and therefore easy to dodge accordingly. If you can consistently dodge Flash Cannons, it will allow you to avoid relobbying entirely while not sacrificing too much overall DPS.
Against Wild Charge, Level 40 Dragon Breath/Dragon Pulse Hydreigon falls a little short of duo-viability on its own, but can be slotted in if you have enough Rayquazas to carry you ahead of the timer.
Against Outrage, Dialga is the only Pokemon that can duo without dodging, even in Wind.
Without dodging, Level 40 Rayquaza dies an average of 102 times against Windy Outrage Zekrom. With dodging, Level 40 Rayquaza will die an average of 18 times.