I control Mystique 6 drop
my opponent attacks with dr. doom (4drop) and Robot destroyer to my mystique , then i flip my Insignificant Threat exhausting my mystique stunning Robot destroyer, does doom go head on into mystique and stun or does he ready??

The attack is still legal and will continue with just Dr Doom.

but with mystic Paralisis the attack is illegal, how is this any different??

As an additional cost to play Insignificant Threat, exhaust a Brotherhood defender you control.

Stun target attacker if its cost is less than the cost of the Brotherhood character you exhausted.

IT isn't stopping the attack. It's stunning one of the attackers, and the other one is still attacking.

With Mystical Paralysis, it is chained to the declaration, so the character exhausts before it attacks.

\m/

oh ok i remeber that. thanks alot