Just a few thoughts stand out from this passage.
Where effort is applied, progress is made. If you apply effort to "solving syllogisms" then you will solve them. If you apply effort "to keeping [your] ruling centre in accord with nature," you'll make progress. He further contends, if you focus on one, you will fail in the other. (see v. 1-4, p. 154-155)
Invincible is another term used to describe the goal of Stoicism. Says Epictetus, "The good person is invincible because he never engages in any contest in which he is not superior. 'If you want to take my land, take it; take my servants; take my public position, take my poor body. But you won't cause my desires to fail to attain their end, or my aversions to fall into what they want to avoid.' This is the only contest that he enters into, the one that is concerned with things that lie within the sphere of choice; so how can he be anything other than invincible?" (v. 5-7, p. 155)
Lastly, every person can convert to philosophy - it's as easy as snagging "soft cheese on a hook" and therefore to find the one who are truly going to embrace philosophy, Musonius Rufus (Epictetus' teacher) would turn students away. Those who refused to be turned away and kept coming back were the gifted ones.