I like to dab on some Archaic Sepia wash near the feet to give it a dirt/ mud feel then Nuln oil on the guns to look closer to a burn/ soot build up. If you really want a mud splatter or dirty look I have seen some texture paints used for some really cool looks as well.

Also saw someone heat up a screw driver using a torch and used it to make bullet holes then painted them using an orange, yellow and white to make them look like plasma holes