As a player who played nothing but infiltrator for a while, I will pass my technique to you. ^^
You should train in the academy (mainly Sword fighting) until it's at least 50-55. This can be conquered really easily as the Academy actually follows a pattern that has already been discovered (guaranteeing a very good chance to lvl up by 1 per train session) experiment with the training levels and times and you will find it eventually, (I spent a lot of time messing with it in Atamara's ancient stalemate), obviously using expert with 10% skill is not advised. It is like a stair case, with each increase, your hours increase ablnd eventually so does the skill level to cut down the time needed. Most people just pour all their hours into expert, but that is truly a waste of money and time in my opinion, I spend less gold in a week and gain at least 2% higher each turn (that is assuming I have no more time for more training per turn).
Now. With your new skill, continue to train regularly until you find yourself at war. Murdering militia is a infiltrator's best friend. It increases your infil skill and your swordskill at the same time. Follow this and you will find your skill to be good ^^ I personally played extremely cautiously because the more bans you have, the more chances you have to be killed. I raised my skills to the high 80's and still found it difficult to do things.
As previously stated THERE IS NO SUCH THING AS LOW RISK. As an infiltrator, you will be crossing your fingers A LOT. I do not know how long I spent staring at that button gaining the courage to actually do it. If you fail, normally you find yourself in even more trouble than before. Most times, you're rewarded with a ban. After that, the risk is at its peak because now you can and will die if you fail in that realm again. Death means all that hard work is now GONE.
Infiltrator is a powerful class that can move past armies and give vital scout reports without being caught for as long as luck allows. However, they are also the very few gateways for death to occur to your character.