i conquer fast, but i almost never attack, i siege cities and wait till another army comes and tries to relieve the city (only if one is nearby). they attack and i simply defend, eliminat both armies and walk into the city. so for that i suggest to use the
10 legionares, 2 auxilia and 2 merc hoplites, 3 ranged units like slingers or archers and 2 (light) cavalry to kill the routers.

if you're a attacking player you should use 8 legionares, 4 spearmen, 5 heavy cavalry units and the rest can you fill in yourself