I think manually is the best way, trying to find the natural accents in a song depending on the rhythm, normally it's accent on the 1st beat and a smaller accent on the third, but that can vary a lot. On timing, it really depends. One thing I do on blastbeats is in some long-ish parts I start dragging the last snare hits a bit behind the grid progressively, to give the feeling of the drummer getting a bit tired and slowing down. Of course, it's all slight or it will really sound like a shitty drummer, but done slightly it definitely helps to make it a bit more human