Jamieson, Fausset and Brown on Colossians 1:29

29. Whereunto--namely, "to present every man perfect in Christ."

I also labour--rather, "I labor also." I not only "proclaim" (English Version, "preach") Christ, but I labor also.

striving--in "conflict" (Col 2:1) of spirit (compare Ro 8:26). The same Greek word is used of Epaphras (Col 4:12), "laboring fervently for you in prayers": literally, "agonizing," "striving as in the agony of a contest." So Jesus in Gethsemane when praying (Lu 22:44): so "strive" (the same Greek word, "agonize"), Lu 13:24. So Jacob "wrestled" in prayer (Ge 32:24-29). Compare "contention," Greek, "agony," or "striving earnestness," 1Th 2:2.

according to his working--Paul avows that he has power to "strive" in spirit for his converts, so far only as Christ works in him and by him (Ep 3:20; Php 4:13).

mightily--literally, "in power."