Matriarch Benezia is a fucking cunt. Just after I make it past all three ways of her fucking irritatingly cheap bodyguards after being stuck for two days, I get to fight her only to die pretty much every time I spawn to take take her on. it isn't bad enough that this bitch has that fucking annoying psi-power that knocks you down to the ground and lets every bitch on the level light you up. I'm a level 29 soldier, and I've maxed out my assault rifle, pistol, shotgun, and soldier stuff, and my party members are Ashley Williams and Garrus. They both spawn with half their health since they both died at the beginning of the encounter. What am I supposed to do in this fight? There's commandos at every turn and cunt Benezia tossing purple shit at me 24/7. At one point I rushed her and managed to take her down, thinking it would trigger a cut-scene and give me a free pass, but nope! Turns out you have to kill every one of her fish-headed super bitches too. Fuck her, fuck the Asaris. /rant.
 
Now, how do I beat her?

I usually shoot her, also I think I use cover when she throws that blue shit at me, however I never had a problem defeating her because her body guards aren't that difficult to deal with. You can probally find a youtube video with a walkthrough, however i'm just still suprised you found her difficult.

Kill the body guards as quickly as possible, try to block her powers and render use the push ability a lot. I think that's how I did it at least.

Clean out that gutter mouth, ha!
 
What I did (for both normal and on Insanity) was to stand in the doorway where you start and concentrate on the guards, they should approach directly from the left first, by the time you have dealt with those you should be able to take care of the other group from the other side.
 
Once you hit the cutscene get ready for the next wave, concentrate on one side on a time or at the very least the commando's closest to you as some often rush you. Whatever part you are on make sure you stay out of Benezia's range, as long as you don't wander around the front of her towards the walkway you should be fine.
 
I'll admit getting hit by her to begin with seems to be slightly random.

Oh man, i remember i almost quit the game because of this fight. I must have restarted like 50 times, i even went to the BioWare forums to see if i was the only one (i wasn't). 

Yeah I remember having a lot of trouble with her too. It was at that point that I realized that I really should be thinking about how I was leveling my Shepard as well as what equipment I was using.

I got stuck in the beginning a few times, i froze and got stuck on some geometry and could not get out.

i remember that fight.. i caused me some troubles too.. i turtled up somewhere and picked off the guards one by one, then moved to another spot and did the same thing.. reapeated until i could take her down..

I just finished finished a playthrough with soldier myself, didn't have much trouble with her this time, I think I did on my original game. Just go around and clear out all the guards before you go near her, as in go right after the first cutscene ends instead of up the stairs to her, once you are sure they are all dead go hit her, she shouldn't put up much of a fight with two buddies there with you.

Sheesh Vinny, clean that post up!

This shit was kinda hard with a soldier, but super easy as a biotic.

Start over as the proper class: Adept. Stasis owns every boss. 
 
On a somewhat serious note - you may need to change your team around, if your Shepard is a Solider there's absolutely no reason whatever to have Ashely with you. You need Garrus and Liara, maybe Kaidan. You want people who can Overload the bodyguard's weapons and Sabotage their shields or use Crowd Control abilities like Singularity and Stasis - basically cripple them and take them out one by one. The absolute hardest part of this fight is the very start of it, you don't have a lot of places to hide or get cover - opening with Singularity on the steps can save your life as you rush to the right. From there it's a matter of staying in cover - keeping the bodyguards at bay as best you can and finally taking out Benezia herself. If Teammates are dying on you, restart from the Normandy and make sure you visit the Medical Bay to get a free resupply of Medigel!

Yeah, probably one of the worst boss battles I've ever seen in a marquee game.  
 
It always ended up being the group of us holed up on that little platform beneath Benezia, trying desperately to pick off the guards when we could, eventually making it to Benezia herself. Very frustrating. 

Remember when biotics were actually a threat in ME ?
And then they all got limp dicks in ME2 boy that made for some EXCITING combat in ME2.

I'm still stuck even after putting the difficulty on casual...I think I might have to start the entire game over, and I was two missions away from beating the main story.

You can't damage her until her Commando's are all gone. If you're trying to pop her while fighting them then you're doing it wrong. Clear the commando's then when you get to the final showdown, pop the power that lets you fire continuously and fill her full of white hot pew pew.

I played as an engineer and brought Ashley and Wrex into the fight, I was stuck at her for so long. In fact I don't think I was stuck at any other point for that long in Mass Effect or Mass Effect 2.

ouch that is a long time for this fight, but when i was playing as a soldier i first killed the 2 biotics coming straight at ya from the stairs, next you go to the right(make sure one of your mates keeps that biotic there busy) 
after that you stand on that walk opposite to benezia so she can't hit you and than you take out the rest, one side at the time, with your sniper (the must have gun for insanity) 
and when every one is dead I think is just rushed her with the shotgun. 
 
Good luck and don't stop sometimes you just need a little luck to get trough ;)