Is this happening on single targets? I've noticed it's hard to target single enemies and just skip them to find bigger mobs. And dealing with just one enemy - as in the Rift Guardian, the Bane of the Stricken gem is a must.
What is your attack speed? You need all the stacks of gogok and vyr 4 piece set to get all the attack speed you can get. Try aim for around 2.0 attack speed i buffed and close to 5 with all the buffs. Higher attack speeds will proc manald heal more often.
Also, you do know the huge damage comes directly after popping into Archon for 8 seconds while the Tal Rasha is active. The rest of the Archon time is just base damage (whatever Archon stacks you have) and whatever Manald Heal deals out. Another good reason to have Zei's gem socketed for more damage.
Thats why the hydras come into play as you now have three sources to proc Manald Heal.
Or to the most extreme you have the COE version which has all the stacks of Archon, COE, Tal Rasha, and Black hole overlapping for those 8 seconds.