I'm not sure what "- mr tricks" is nor what it has to do with your post so maybe I'm not sure what you are asking here.
If you are charming for exp you do generally want to charm the lower level of 2 different mobs, but usually not "green versus blue", but more "low blue versus high blue".
Lower level will mean charm holds more reliably. Lower level also usually means you will have to do some significant damage to the higher(non charm) mob. But that's easy for a necro to do especially if its damage over time, so you can use your highly mana efficient dots at the start of the fight and by time the fight is getting close to over the dot will have mostly run its course. And lets not forget that green mob will yield much less exp than a blue.
As far as keep your green charm alive versus a significantly stronger blue con your choices are:
Fear(usually paired with drkness)
Just doing more DPS with nukes/dots, a mob that dies faster does less damage
Drain tanking(usings your life taps while "splitting" some of te damage between you and your pet. Not the best idea to be doing this when charming but it is what it is.
Healing your pet (variety of ways to accomplish this).
Giving your charm pet various proc weapons that will either do more DPS or reduce incoming damage, like a slow proc.
If it's just the case you want to keep the green charm alive so you can continue to use it, you can always just FD and wipes it aggro s it starts to turbo heal and then recharm when its life is back up.
You are really not clear in your post at all what you are talking about so I think I covered pretty the majority of stuff you *might* be talking about.
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