The level Cortana in Halo 3 is not impossible alone on legendary, maybe with all skulls on I'd agree with you or even just blackout would make it nigh on impossible alone. However you do die a lot and learn a lot about what weapons to use and not use at different times. For example with flood it doesn't matter what guns you have so long as they have as much ammo as possible. I tend to aim for plasma rifles and swap the lowest charge one out every time theres another up for grabs. Keep pressing forwards and you can eventually win through. Also, against flood the hammer is far superior to the sword as far as melee weapons go, learning that has saved me from death so many times, the flood love to well flood you with too many numbers for survival purposes just hammering can push the crowd back and kill some. The first time I ran in to a level what required me to pause for a couple of weeks to figure out a way through was the Ark where I ended up with the two hunters as you work your way inwards and I was low on ammo and grenades, in the end however with some inspiration from how Psy tends to challenge them I won.
Hardest achievements I've gained? I didn't bother with Mile High Club on CoD 4 but I have every other achievement in that game, I could grind out the Mile High Club but well, for 20g is it really worth it? I also completed MW2 on Veteran too. These achievements are hard to get because CoD can be really tough especially if you make a mistake, on Veteran sometimes if you cock up slightly it's best to either die or re-load the check point to just correct it.