Can my opponent use manical Laughters E: to blow itself up so he can gain a monetum during MY attack? the effect doesn't say only playable buring your attack but I am not 100% sure and I would like to know this

Yes, your opponent can do that, because the "destroy a foundation" part is a cost. The ability does nothing, but the destruction occurs.

-Tinman

Yup, he need only pay the cost.