Heavy machine guns do quite a bit of damage to them so if you keep your mg42 paired with a gren or so while posturing a flank you can at least push back the flack halftrack/ M8. This buys you time to get out a pak or Panzer grenadiers with shrecks. If you have a 222, you have two viable gambles. You can either bait the light vehicle through great positioning or you can keep it in reserve only to pounce if you get a faust off and the Mg is shooting at it.
Vet one machine guns with AP rounds kill it off in 2 bursts so that is also another way to push it back.
Mines are another option, US players have to kill you in a reasonable amount of time so it forces them to be super aggressive with their light vehicles as well. This means generally the light vehicles will be traveling from map end to map end via roads, so Tellers are your friend.
In terms of area denial S mines also do tons of damage to light vehicles. You can use these to limit the flanking routes around key areas you are holding.
The goal here is to delay, good players make it hard to secure a kill on these but after you push it back the first time your pak or panzer grens limit the effectiveness of the light vehicle. Soon you should get out a StuG as it is your fastest hard counter that will secure the kill or render the M15 basically useless
Here is a link that demonstrates how powerful the MG42's armor piercing rounds work against the M15
http://www.coh2.org/topic/24602/don-t-mess-with-mg42