There is no specific skill or a particular rank of a skill or that you have to have a specific skill at a certain Tier. When you have enough skills trained at whatever ranks, such that your combined skill "points" are a certain number, then you will survive the center squares. I don't know what the actual points value is. You either need a lot of low level skills, or you need a few high-ranked ones. What number represents "a lot" or "a few" is unknown to me. On the server displays I would see it as you being Tech Level 6. I don't know if we display that value anywhere on the client or not.