Fathom is alot better than Knowledge, if infinity cant break the immune, chances are knowledge won't either. You can always use knowledge on switch as a "just incase" weapon.
Your choice on diamonds vs facets. I would probs use diamonds simply to make better use of the counter shields that proc alot faster than you can cast on a single target boss. If you go the defensive route, then make your merc your offensive side (infinity). if you go the offensive route then you might want to use ice and medusas on your merc to do the main tanking for you.
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