How to make a hostile mob follow you anywhere and everywhere forever until it's killed

The bad sheep bossfight I'm making does not take place in a special area, but rather the actual main part of the map inside the house and in the yard. I have the followrange set to 2048 for both the sheep and the invisible/silent vindicator riding the sheep which makes the sheep follow you to look like the sheep is hurting you. However even still then the sheep will stop trying to find you if your like in the room below or too far away. How to fix this and get the follow range to actually work properly?