you should really get a well balanced army and then make a strategie. i never field an army without thinking about wich units i shall put in.

use this 8 hastati (or other pilum throwing infantry)
4 merc warbands (or other spear inf)
4 velites (or other missile units)
the rest is upt to you

field it like this. 4 hastati (1)
2 spearmen 4 hastati (2) 2 spearmen on the flanks in a diagonal line
4 missile units

and the rest depends on the units you pick. this way you'll cream every army even when it outnumbers you 1:5