"Effort" as a noun means "an attempt". Here, it's just another word for a kick.
"Squirmed" is unusual here, but I can see that it means it squeezed its way past the goalkeeper (I assume a goal was scored). It wasn't a clean, direct goal - the goalkeeper probably got quite close to the ball but still couldn't save it.
"On the halfhour" means it was 30 minutes into the game.