Took literally half an hour just to get the Cardinal to half health on Insantiy

I gave up on Insanity for the same reasons.

Fights aren't "hard". They just take ridiculous amounts of time.

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

That and the mobs respawn every so often.  Its the same with all the ascendant, you kill all the mobs then focus on the orb until its gone then hit it with a fire combo, rinse, repeat.  It would be faster if your squad members were smart enough to combo the orbs.  Its faster with a shotgun at point blank but then you'll only have a second to dodge the energy balls.