Just using the original playtest material, and some of the revealed content that was unlocked during the playtest, I'm going to put forward a way to focus on using The Lost spirit for the medium class.
There are two things that make The Lost less than ideal for play: it's not inherently a Strength spirit, and it's mechanic revolves around Confusion.
The Lost's Mad World ability only gives you a slightly better than 1/4th chance for it to become a Strength spirit, but you need to be right next to the creature you intent to attack, otherwise you'll end up attacking someone or something less ideal (like a fellow PC). As the rules stand right now, you only get +X to the Confusion table roll equal to your Wisdom spirit bonus; at lower levels the benefit isn't going to help very much with determining the outcome. At the end of the day, you're more likely to attack the nearest creature and less likely to act normally; the chances of babbling or hurting yourself stay the same. The biggest downside to this ability is that it doesn't mitigate the potential for 1D8+Str mod. damage to self. Technically, it says: Deals 1d8 points of damage + Str modifier to self with item in hand. If we take "with item in hand" to its RAW conclusion, if there is no item in hand, you don't take any damage and babble instead. We'll look at it both as written and as some have interpreted it otherwise (you do the damage no matter what).
If the GM rules that you take the damage no matter what, or your concept is jonesing a weapon, then the easiest course is lowering your Strength, pursing the Dex-to-hit and Dex-to-damage feat lines. Your damage output will be consistently sub-par until Lv 5., where you can channel a two spirits, and Lv. 6, when you can get Slashing Grace/Fencing Grace (having taken Weapon Finesse at Lv. 1 and Weapon Focus at Lv. 3. You'll also need to take Martial Weapon Proficiency at Lv. 1 if you're playing a human and you don't want to get stuck with the sickle or dagger, but you might prefer Spirit Specialization for a boost to Strength). The prowess of this build in the first four levels can be improved by making The Cyclone your primary spirit, since you wouldn't be able to cast confusion effects until Lv. 4 anyway, and only once or twice a day at that. Personally, I wouldn't go below 8 Strength, which brings the 1D8+Str mod. to 0-7 damage. Still painful, potentially lethal at lower levels, but further mitigated by beseeching a Con spirit and running The Cyclone early. [If you love scimitars, Dervish Dance is obviously the way to go; giving you access to Dex-to-damage at Lv. 3.]
If your GM takes it as RAW, then you can go through the unarmed feat tree, or roll a race with natural weapons, like the Skinwalker, and not worry about taking the damage at all.