"be awake and alert" or "watch out." Christian leaders being alert about anyone that may harm the community of believers is spoken of as if they were guards in an army watching out for the enemy army. (See: [[rc://en/ta/man/translate/figs-metaphor]])
This does not mean that the only words he spoke were words of warning. Rather, Paul is using hyperbole to remind them that he had told them often that evil men would try to deceive them. Alternate translation: "I warned ... you as often as I could" (See: [[rc://en/ta/man/translate/figs-hyperbole]])
Here "tears" refers to Paul's crying because of the strong emotion of concern he felt while he was warning the people. (See: [[rc://en/ta/man/translate/figs-metonymy]])