how many levels are there to the campaign? and should I abuse the skirmishes to level my characters?

There are tons of levels. You don't have to do skirmishes but it makes battles much easier, and allows for a bit more wiggle room for mistakes.

There are 18 chapters, but there is more than one battle in some chapters.

And some of "reports" you can buy from Ellet (the journalist) will also contain side missions.

@ sieghardt

Thank you! And only $30!

Chapter 7 help plz :3

if that is the one with the extremely large tank then I'll do my best. First goal should be to kill the turrets on the big tank as well as killing the troops on the cliff using a sniper and perhaps an assault. You should probably have atleast two lancers ready to take out turrets on the tank. Once the radiators show up it'll be easy to run lancers up the ladders and take them out.

Basically you only need antipersonel troops take out two waves of enemies, everything else is dedicated to taking out that tank. It's useful to have an engineer to restock your lancers once they get low. Dealing with Silvaria basically amounts to trying to make sure that noone that can die is within her eyesight (so only a tank can be within her eyesight).

Everything goes fine... until Selvaria shows up >_>

I've tried three times and I can't seem to keep my units away from her long enough for the tank to fire and expose the last radiator. Then I call some reinforcements, but the Edelweiss gets destroyed before they even get there >:(

what's destroying the Edelweiss? Is it getting in front of the super tank?

Well regardless, once Sylvaria is around you need every person to be out of sight. Hide your lancers in the trench on the opposite side of the map that the supertank will pass by. Anyone that's out of that trench should be behind your own tank. Really it's just all about keeping everyone out of her sight, no matter what it takes. Sylvaria will usually stay around the supertank or your tank (during my matches my tank was always near the supertank) so her movements are usually easy to predict, and if she can't see anyone then she won't even use her movement.

Supertank has an indestructible turret at the rear, too >_>

Well, I beat it, anyway. Lancers kept dieing before they could even get a shot (which misses 99% of the time >_>), so I just filled the trenches with shocks and snipers and got some scouts to climb the ladders after Welkies dealt with the turrets.
Selvaria thought it'd be smart to guard the last ladder, so I ended up having to make a barrier (with my tank) to block her shots while Alicia 'naded the radiator :P

The key there is to collapse walls to trigger faster shots from super heavy tank - that way you can destroy 2 radiators before she arrives.