From my understanding, critical hit damage is based on the hp that the target vanguard has left, so say in your screenshot it's near the beggining of the game and your vanguards are still full on hp. A vanguard's hp is about 300k-400k depending on level, your screenshot shows that he did a 4 card critical attack, thus doing instant kill dmg dropping 4 of your vanguards. The dmg would be equal to their hp they have left, so about 1.2 mil dmg, it seems about right. Doesn't look like a hack to me. Critical hits don't take into account the cards that they used, only the hp of the target vanguards.
Same goes for OP's photo, he hit all 5 vanguards, thus knocking the total of 5 vanguards worth of hp. which is about 1.5 mil.
Edit: This is based on the critical hits I've seen, the damage they do is always in approximate range of the target vanguard's hp, and with the hp boost we've gotten at beginning of month, the damage is huge.