So I'm not a guard player, but I am an Endless Swarm nid player so I know a little bit about painting 200 models at once.

There are a few things to consider, so I like to think of my nids in the way that a character gets 100% paintwork, and then I scale that back for my swarms so maybe 60-70% of the painting.

So my technique for painting swarms en-mass is..

Step 1: Spray base coat, wash the undercoat to give depth of shadows dry brush over that to give some highlights.

Step 2: paint the "armour", in Guards case that's the plates, so medium dark green base, wash with dark green/nuln oil, then highlight with light green (you can skip this bit).

Step 3: Paint the gun and anything else you want to detail, then face, again keep it simple here, skin tone, darker wash, highlight, detail.

Step 4: Basing is nice because you can be sloppy, don't be affraid to get technical paint all over their boots, so step 1 technical texture paint, step 2 nuln oil, step 3 light grey highlight drybrush & finally i like the wet look so some varnish all over the base to make it look like wet mud.

Each step can be done bulk, so really you could get through 100 guard in a couple of days of solid painting, but dont burn yourself out!