Skirmishers are typically ?/W class combos which generally lack the endurance to stay in melee like a warrior primary can, but still provide a useful service in combat. Skirmishers need not be associated with warriors at all, and might simply make use of spells or skills which require touch range to activate. The root word skirmish is defined as a minor fight incidental to a larger movement, or a small contest between two parties. As such, skirmisher tactics revolve around secondary or tertiary tanking, harassing tactics, and spike damage.
Secondary/tertiary tanking is like sheep herding. The idea for secondary tanks being to block monsters from softer targets and direct them towards the primary warriors, or serving as a means of penning monsters in for AoE spell damage. Tertiary tanks serve the role of "mage guard," protecting against targets who make it by both primary and secondary tanks. They are important for party survival in mob situations, but generally are the least important builds and are considered optional especially late game where non-warrior primaries can take far too much damage to tank at all. Warrior stances and certain other skills (such as Blurred Vision) are staples of secondary/tertiary tanks for late game survival.
Harassing tactics revolve around specific close combat skills for fast, influential effects. Thunderclap or Glimmering Mark and a lightning weapon, Ward spells, Mark of Rodgort and a fiery weapon, or Earth Shaker/Aftershock combos are all widely successful as harassing build combos. After completing their task, these skirmishers retreat to wait for their combo skills to refresh and lessen health loss. Harassers are more popular in PvP than PvM, but are useful in either situation.
Spike damage melee builds usually revolve around unleashing a torrent of spells and skills in close combat quickly, then retreating to recover lost energy and/or health. For example, a necromancer with Dark Aura and a variety of touch and/or sacrifice spells is potent in close range (mainly due to Dark Aura's area effect when triggered by sac spells, Touch of Agony in particular which can easily exceed 100 damage combined which bypasses armor). After a quick run of the combo spells, when health and/or energy is depleted, the skirmisher retreats to recover for another run.