Report All Activities by Account in Regarding Rollup

Hi guys,

I have a very special requirement and I need help or ideas, my platform is MS Dynamics CRM 2015 OnPremise:

My client needs a report of all the activities where the regarding to: has as an account associated with a certain client.

Example:

Activity A:

- regarding to: CASE A

Activity B:

- regarding to: OPPORTUNITY B

Where within CASE A has an ACCOUNT XXX

Where within OPPORTUNITY B has an ACCOUNT XXX

I need in a report the activities A and B filtered by the ACCOUNT XXX that is in each entity of regarding to, in the activity.


It's like a Rollup of activities, but filter by account contains in each entity. 


I appreciatte any idea or help.


Thanks