Heden,
The game is not so hard as it looks like. But when you start this game its best just to have some fun, and learn how the game works.
95% of all battles are won by using the rights spells and troops.
So best advice is replay some battles in the beginning and try different troops and spells.
Most players play a mage with 1 tank unit and a bunch of support units.
Spells as Fear , Slow, Stone Skin, Magic Springs are great.
Also summon Phoenix is great spell till lvl 30, But you need the lvl 2 or 3 version.
Download the appendix to give you some help.
http://forum.1cpublishing.eu/showthread.php?t=12249
This is for Armored Princess , but works also for Crossroads.
Also take a look at the following tread to learn more:
This one if for the first game but most of the info still applies to al the games
http://forum.1cpublishing.eu/showthread.php?t=10311
http://forum.1cpublishing.eu/showthread.php?t=15448
http://forum.1cpublishing.eu/showthread.php?t=11306
http://forum.1cpublishing.eu/showthread.php?t=11218
This should help you on the way.