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As King Arthur, cast Excalibur's Wrath and Sundering Strike within 10 seconds of each other.

Excalibur's Wrath and Sundering Strike are King Arthur's ultimate abilities. Arthur is able to switch stance between his standard abilities and his combo abilities. You switch between them any time you use an ability.

Sundering Strike requires 35 energy before it can be used, and Excalibur's Wrath requires 80 energy before it can be used. You gain energy by hitting enemies with your other abilities. Your 1 will give you 3 energy for each hit (so 6 energy in total if you hit both the standard stance and the combo stance). Your 2 will hit for 4 energy each (8 in total) and your 3 hits for 1 energy in each stance.

The best strategy for this would be to build up to 100 energy first (this is shown in the sword icon that appears for King Arthur's passive (it appears in the bottom left of my screen). Once at 100 energy, you'll want to go on the offensive. If you die, all of your charge is lost.

Both your ultimates have a bit of a dash forwards. Use your ultimate Excalibur's Wrath to knock an enemy into the air and strike them 6 times before dropping them back to the ground. You then have to build up 15 more energy by attacking enemy gods with your abilities and then hit another enemy with Sundering Strike, all within 10 seconds.
You'll be at 1 stack of passive. Your 3 in Combo Stance (where you spin in a circle for a few seconds) gives 2 energy for each tick hit so you'll gain around 12.5 energy by hitting a god with each tick. Hit one another ability or two after this finishes and you'll get the 15 you need. Cast your other ultimate, Sundering Strike, and the achievement should pop if you did it in time.

This is best attempted in Arena. You'll want a standard tanky-warrior build, Cooldown Reduction can help with how quickly you can use your abilites but aren't a necessity. Protection are a necessity however, as having your energy reset can be a pain, so you'll want to build items such as Gladiators Shield, Void Shield, Genji's Guard and Mantle of Discord (and any other situational items you think are necessary.

You don't have to hit the same god with your ultimates, and it seems far easier to hit someone with Excalibur's Wrath First, then go for Sundering Strike, rather than the other way around.

I'll add my specific build in once I've earned this achievement myself.

Thanks to fifapro for the additional information.