Not gonna lie, i am gonna say, you must use a different strategy
I am a normal difficulty player
Here is my setup
1. Black Widow
2. Valkryie
3. N7 Eagle
The first gameplay, i was using Biotic Combo
So i crushed the orb with Widow / Valkryie
Then i combo him with Pull --> Throw --> Lance
Took me about 10 minutes (maybe 15) to kill the cardinal (Killing the orb maybe 4 --> 7 times)
Then in the NGP
I changed my strategy
Weapons : Isharay , Valkryie, N7 Eagle
2 Profiles:
Sentinel with Overload , Singularity, Energy Drain (In other words, Shield destroyer...)
A moment before the orb die, i changed to my other profile
Infiltrator with Omni Grenade, Turbocharge, Tactical Cloak
----------
Its all about timing
After you give some hit to the orb, don't kill it yet
Prepare for a super attack right after the orb dies
Turned on my Turbo Charge, Emptied my N7 Eagle (400 damage, high fire rate + Turbocharge), then i changed to my Isharay (still with turbocharge) while Tactical cloack (Damage Up), Hit the head
Guess what? Same difficutly, same level, under 5 minutes. Only 2 orbs needed
So...
TL:DR
You need a better strategy for the Cardinal
There is some guy here, using Insanity difficulty too, and he didn't complain at all about cardinal..
So... Sorry to say... you were doing something wrong
