I define a skirmish mech as a Battlemech that has a balanced mix of armor, mobility, and long/short range weapons for a reasonable battle value- usually light and medium mechs.
A skirmish mech operates alone, or a bit ahead of the advancing main lance to fire at targets of opportunity, and try to tie up or slow down advancing units, either by driving them into cover/stop moving, or to break formation to deal with the skirmish mech- thus breaking apart the advancing formation so it is less effective in terms of unit support and overlapping lanes of fire.
A good startiing skirmish mech?
The Shadow Hawk.
Good mobility in terms of speed and jump ability means it can get into position, plink away, keep up or run away as needed.
Armor is solid for a medium mech so it can take a PPC hit or two and not be crippled.
The AC 5 and LRM 5 have a long enough range- with both working together to engage targets at distance, and if an opposing unit comes close or moves to engage, they Sahdow Hawk can lay down the AC 5, SRM 2 and medium laser fire as it moves away.
All for ~1064 battle value which is solid for a medium mech.
Other Battletech skirmish mechs?
Check out the Valkyrie.