Not clear what or who the subject of these sentences is
有些人[你]不必理会. 你 is dropped as the context is clear. If you read the sentence again, it starts with:


  当你长大，你慢慢学会几件事情；有些人不必理会...


Chinese is a pro-drop language, you will encounter this quite often. 理会 here does not mean understand, but to bother or care about. So, it is saying, there are some people that you don't need to care about.




  有些人永远成不了你的敌人，[你]不必抬举他，这种人太看得起自己了，以为拼命攻击你伤害你，就能够被你所恨。


Some people will never become your enemy; there is no need to flatter them. This kind of person thinks too highly of himself and assumes that by attacking and harming you relentlessly, he will be hated by you.

There is no double meaning here for 敌人. It is just a philosophical statement advising you not to bear grudge against an unworthy opponent.