I found this trick video to be worth sharing, its basically all about Getting on top of Last Resort! :)
Also here are som tips about grenades in Halo3: Some are simply unaware that when your running forward and throw a nade, that it goes further than if your standing still or running backwards. So the grenades have a momentum behind them, if your running backwards you have to aim higher if you want them to go far at all.
It's a good habit to throw random nades, what I mean by this is throw nades where you think an opponent may be although you cannot see him, or if you see an opponent running behind a wall and you can throw an anticipation nade.