When I say trench, I don't mean an actual modern trench-like what you see appearing in the 1860s, those were around, but usually only for artillery batteries, I mean like one of those trenches where you dig up behind you and place the dirt in front of you with either spikes/twigs/extra stuff to form a little embankment, which was usually enturned with redoubts, rountles, barrettes, and spikes. Which is usually when there is little to no time to create anything. 

But going on about Elite troops, most elite soldiers are just shock troops or used as reserves in the last resort effort of a failing battle/something where a little bit of scare tactics could break the enemy. Napoleons three-guard units were rarely seen in battle fighting, most of the times they fired shots just out of viable range and marched forward in the smoke to scare the various troops on the other side. The two times I know they fought were Borodino and Waterloo where they were as effective as a better regulars unit.  Well except waterloo when they were watered down a bit.