These are probably the easiest missions but often take the longest time to complete. The largest type of missions and the ones that give you the most choice, in these missions you have access to everything at your current tech level.
Some of the most difficult missions in the game are Skirmishes largely due to the fact you are constantly under siege. Unlike the other missions, Skirmishes require you to work fast and get things done quickly and efficiently. You have a base this time but the only difference is that you lack a Construction Yard (or Crane) which means you are stuck with what you've got. Skirmish missions are the intermediate step between the small scale engagements of Guerrilla missions to full on wars of Battles. In most missions there is room for personal choice or error but in order to succeed in these you'll need to follow my instructions to the letter. These are a personal favourite of mine but I understand most people despise these with a passion.
Slow and steady is the key to victory and saving regularly is the most important thing to remember, more so than on any other mission.
These missions require the most patience out of all of them. Sometimes you have just a single unit (a commando) whereas other times you'll have a sizable force. Guerrilla missions are ones where you have no base and very limited resources and troops to complete the mission. This is one of those love it or hate it things.