Mercs can help in the early game because they don't cost extra resources, just gold. Other than that, it depends on your nation, but most starting armies can do pretty well against indies on the default settings (independent strength = 5) if you just toss in 10-20 missile units. A key point: use your Prophet or other priests who can cast Sermon of Courage or (even better) Sermon of Courage + Smite.
Beyond that, it's hard to get specific without talking about a nation and/or pretender build. Here's a link to a DAR (During Action Report) that I just started:
http://bluishcertainty.blogspot.com/...-part-one.html
You'll see that it's early fall (Turn 6) and I own 7 provinces. I did take a good bless (E10N6) and one tough province (knights) was conquered using a blessed Adon after my mercs got slaughtered, but 3 of the 7 provinces were conquered using the starting army + Prophet + 1 turn's worth of slingers, plus one Ahiman Anakite. One province is of course my capital, and the seventh province was conquered by the aforementioned mercs on the third turn before they got killed against the knights and heavy inf. (Turn 1, Prophetize starting commander and send out scouts. Turn 2, look at intelligence reports and decide who's weak, send out troops. Turn 3, battles actually occur. In this case I took 2 provinces on Turn 3, none on Turn 4 (picking up troops) 1 on turn 5, and 2 on turn 6.) I'll probably try taking at least one territory per turn with both of my current armies until they suffer some kind of disastrous defeat, and build more armies at home in the meantime. One thing that can slow down expansion is getting too worried about casualties, and thus trying to build up overwhelming force before you attack. It's usually not necessary, and you can skip tough provinces in most cases and/or leave them as perimeter guards.
Really, most indies are pushovers.
-Max