I would describe myself as a casual player too, sometimes I'm good at combat, sometimes I suck. As long as I'm having fun I'm not that bothered.
For ME1 I had a lot of trouble with my first 2 playthroughs. As an infiltrator I could snipe and use the pistol which suited me, that's the class I used for Pinnacle Station and cleared the whole thing on Normal mode. Then I played as a vanguard and sheesh I sucked at it. Running in with a shotgun - not my strong suit apparently, it was so hard I skipped P.S. and played on easy mode.
Just done my third run as a Sentinel and I'm loving it, I can snipe and use a pistol like an infiltrator but have more armour and survivability. I still didn't re-do P.S. though as honestly it bores me. The prizes aren't worth it and in ME3 you get a new arena mode that is infinitely better.
So honestly I'd say walk away, play the rest of the game and forget it exists. The fact I haven't even missed it my last 2 runs goes to show how meh the whole DLC is. It was only created to give ppl more ME while ME2 was being developed anyway, it has zero impact on the story going forward (unlike Bring Down the Sky which is referenced later.)