The best feature you can use the higher you Elo up is the speed of the unit. Making the Militia’s line faster would make them much more manoeuvrable, because I was talking about making them 0.96 (+ squires) like archers.
In this case we would have:
0.96 Archer = Skirmisher = Militia
1.00 Sperman
Of course the Celts’ bonus should be recalibrated, because it was already nerfed once.
Other highlights that it is a problem of pure unit speed; not so much of HP or attack.
Viper once suggested instead reducing the collision size slightly or giving it something like 0.3 range