Block CH on ALL group members and pets. A merc will not duck out of a heal to cast a faster one. Once they press that CH button on any group member or pet nothing is more important to them than their nails.
Block Promised on tank.
Merc healing will improve dramatically if you and your group will do those two things.
.....For my toons i have to block Complete heal+ PR, + elixir's(HoT's). THough i will unblock them if i have a real healer in group because i know a real one will cast spot heals unlike the merc's who do not.
http://forums.station.sony.com/eq/posts/list.m?&topic_id=181887
http://forums.station.sony.com/eq/posts/list.m?&topic_id=181434