If my opponent attacks my controlled arena with two tokens and plays Imperial Decree, does he kill both my 6 might units there?

Ruling: No, Imperial Decree will not kill both units when attacking with two tokens simultaneously. You cannot assign damage to a second unit until you've assigned lethal damage to the first, and Imperial Decree creates a trigger when a unit takes damage that kills that unit (rather than making any non-zero damage lethal).

Nuances:

• If the tokens attack one at a time instead of simultaneously, then Imperial Decree would kill both units