Underpass roof mischief
Land on the underpass roof (maybe even on the pinnacle) and wreak havoc on the enemy that gather over yonder. In particular you've got great opportunities for setting off huge chain reactions, as the enemy tend to concentrate in a particular spot. You can keep killing them with a sniper rifle or pistol, then eventually make things go bang by sending in a grenade or rocket. This is only mischief rather than a proper 'battle', but it's still fantastic stuff.
Watch your heads!
Something else I like to do here is rocket a Shade or Ghost about, trying to get it to land among enemies and squash them. Watch them dive frantically to get clear. Hee hee! I often leave a spare Shade or Ghost in the vicinity, with this entertaining sport in mind.
Where did he go?
If you crouch behind a stationary shield, you can enjoy watching through it as the enemy loses track of you and Grunts go creeping around. Should be fun to lob a grenade into their midst, to wake them up a bit!