Complex sentence: He claimed his 25th kill, a B-17, on 9 October, and then served with II./JG 300 by early 1944, shooting down a B-24 bomber on 21 February.
Split into simpler sentences: He claimed his 25th kill, a B-17, on 9 October. He was then transferred to II./JG 300 in early 1944, and shot down a B-24 bomber on 21 February.