OK I decided to reveal some "secrets" here, as I am a bit bored and don't know what to do atm ... So here is something that can even help Gatts improve his play
These I read from the russian fan manual (I am lucky to be able to read russian lol), and I have tried them, and they work pretty well.
Before I said XP has nothing to do with turns, and this is so. But what is the thing that determines the amount of XP then? Actually, two or three things that have not been mentioned yet, but certainly they are the most important.
1. The amount of the total leadership of the alive enemy army remaining when you use the skill. So if you use a rage skill when the enemy is "almost" destroyed, you will get "almost" no XP for your spirit (or zero). Better use the skills as soon as the battle starts, when the enemy army is still intact, and first use the spirit that you want to level faster.
2. The number of uses of the same spirit in the same battle. Each subsequent use of the same spirit reduces the XP gains, and usually after second use the XP gained is already insignifficant or close to 0. The conclusion is - for best total XP, use each of the spirits once, then use the first one second time, if the battle isnt over.
3. The level of the skill that the spirit uses. Higher level - more XP. For each skill there is one type of upgrade that is considered "main". For the damage skills it is usually the upgrade that increases the primary damage of the skill (and the cost :S).
4. If you use a skill that causes damage, the more and the higher level of the creatures killed by the skill, the more XP he will gain.
5. The relative level of the spirit to your hero level - if your spirit level is below the hero level, he will get XP bonus, if he is higher than the hero, he will get XP penalty. This helps the third and the fourth spirit to level faster at the start.
First 2 of these things are certainly the most important. And if you know them it will help you level your spirits faster than the XP talent, as I said before.