More fun with Grunts
Here are a few things you can do if, like me, you love the Grunts' amusing behaviour (you can find some other bits of grunty fun in some of the level-specific sections). It probably also helps if you like those wildlife programmes on the TV. Gun-toting knuckleheads, please look the other way for a moment while the rest of us have a bit of fun; I'll be back with you shortly.
Hide 'n seek
All you really need for this is a Grunt and a rock. You can hide behind the rock, listen to the Grunt's various grunty noises, let him look for you around the rock, and keep moving around as his clip-clopping comes nearer. After a while of intermittent running around, he'll probably start creeping off with that wonderful crouched animation, as if he thinks he's some sort of deadly predator. You can then sneak up behind and have fun making him jump out of his skin by firing a shot past his ears; possibly followed by a stiff whack to the noddle, unless you want to run off and play some more. Call me a softie but I can happily play this for ages (at the end of the rockslide megabattle for example).
Right behind you
Run close behind a panicking Grunt, right up against him, so he fills most of the screen (it takes a bit of practice). It's hilarious. You get lots of opportunity for this in the rockslide megabattle.
Too close, too close
Sometimes when you whack a Grunt and he survives, he'll say something you otherwise don't hear very often. For example, it was a long time before I finally heard a panicked "Too close, too close!". Another rare exclamation was "Do dat again! Ooh I dare ya!". So, try it out and see what you can get. Incidentally, when you whack a Grunt and kill him, look at the blood pattern on the ground. Have you ever noticed that it's comically Grunt-shaped? Looks like it to me at least.
Safari
Well away from your subject group, use a sniper scope to watch from afar as they creep around and suchlike. E.g. you could do this with the 'lifeboat group' in the rockslide megabattle. If there's an Elite around, you might like to shoot him to make the Grunts panic.
Grunt gaggle chaos
When you've got a gaggle of grunts, move around trying to make them shoot each other accidentally. Listen out for the relevant phrases such as "Watch it!", "Sorry!" and "It wasn't me!". It's hilarious. When you're down to the last Grunt or two, the obvious thing to do would be to lob a grenade to send everything up in a huge explosion, bearing in mind all the plasma grenades that should be lying around by then. Excellent stuff, especially on Easy. Even if you only have two Grunts though, you can still have fun trying to get one killed by the other, though it can take a bit of perseverance and footwork.
There's an excellent gaggle you can arrange at the start of level 4; see Grunt gaggle fun for details. There are also some good indoor opportunities in level 5 for example. If you're willing to put in more work, there are some extremely large gaggles you can obtain, including one of 32 in the dustbowl megabattle!
Needler overkill
It only takes a handful of needles from the needler to send a Grunt up in a nice pink explosion. But I find this far too restrained. I can exclusively reveal that the correct way to do it is to send an entire clip into him, as you close in at speed. There, now doesn't that feel good?
Death by Grunt
Unless you're frankly hopeless, you've probably only rarely suffered the indignity of being killed by a Grunt. But it's worth letting them kill you occasionally, to hear the funny things they often say afterwards. My favourite? "In da face!".