You really should use multiple study resources but Destroyer is, without a doubt, the best one (that and Crack DAT PAT). I went through it thoroughly five times and ended up with 23s across (23 ochem, 23 gchem, 23 bio). Bio and ochem I was already pretty good at (only wish I had known about the roadmaps sooner though...could've used them for my actual ochem class) but Destroyer really drove gchem home for me. I also watched about 3-4 of Chad's videos for the gchem topics I was worst at, but I didn't use them as much as other DAT studiers might, I don't think.
And yes you should definitely focus on the broad picture first. I just think Destroyer was great for bio too in the sense that it applies the broad picture to difficult scenarios (e.g. not-so-straightforward Hardy-Weinberg calculations) that, on the test, could be that one question that so few people get right that it is worth more than the other 'easier' questions are. Good luck
