Bellyflop marathon
Any time you're in a Banshee and there's a Grunt or two below, you can have a hilarious time flying right down to them and slowly skidding along the ground, making them repeatedly do amusing bellyflop dives to get clear. A brief thrusting motion just to one side of a Grunt will make him dive the other way, which potentially enables you to herd him in a particular direction if you want; e.g. towards a big drop, heh heh! There's a bit of a knack to it and you'll need a gentle touch on the controls, but do it right and they haven't got time to turn around and use those plasma pistols or needlers; it's just dive after dive non-stop. I've got dozens of dives out of them, but I usually lose count after laughing too much.
Opportunities
There are good opportunities for this excellent sport if you've got a Banshee early in the game (such as stealing the platform Banshee). One way I was doing things was to arrange to have a Banshee waiting for me as I entered the first ground battle, as described in New toys in the first ground battle. After using it to kill all Marines and all covies except one Grunt, that left me with a nice large expanse in which to put the unfortunate grunty survivor through his paces.
There's also good scope for the bellyflop marathon business when you're coming back the other way, in Two Betrayals.
It's a record!
Stop press: after further practice, I managed to get a 112-bellyflop marathon out of one grunty fellow in that first snowfield, before I accidentally squashed him. Amazing how much you can get out of those little legs!