Lv145 int caster, just returned to DLC after 1.5 away, died to Blackgoal knight ~250-300 times, dancing lion ~25-30 times, Renalla ~20 times. After all these deaths on my last try I switch summon to Lv 10 Great Shield skeleton dudes and use 10 Carian Regal Sceptor and a Silver Mirror shield... and used good o Rock Sling spell.

the Great Shield Skeletons are the MVP - they gang up on Renella, restricting her movement (to attack you) and making it much easier for the slow Rock Sling to hit, which causes stagger after 4 or 5 casts (I think). Just gotta watch out for her ultra long & wide spell slashes .

When her health is low, watch out for her moon balls, making sure your HP is high, and have stamina. When she jumps up, hold up the shield and I was able to block 2 hits before stamina runs out? and that 3rd hit almost gets me. By then the skellys are dead but I was able to kill Renella with the NPC cooperator at 5% health I think. That was the 1st try with the skellies. Did not get nearly as far with mimic nor tische black assassin summons.