Merlin Mid (lmk how build worked so i can do my best to improve the build)

I have to agree w/ Kriega here. While the items you choose are generally okay considered individually, % pen is a core stat for any damage dealer and it should not be underestimated. It's even more important with the TTK adjustments HR has made to the game (including base increase in protections) a few patches ago.

There have been times in the past when Book of Thoth and Warlock's Staff were both in very strong spots, and short durations where double stacking items have been viable. In this case, especially when getting both means you delay core items, it's not a good idea.

As you see from Kriega's build examples, Obsidian Shard is being built in the middle of the build after a couple of other core items (e.g. providing CDR, flat pen or other). Your main example build does have some % pen early via Warlock's, but doesn't get Ob Shard until last item, which IMO is too late. The % pen means you'll bypass more of an enemy's protections (inherent or from items), which allows other damage such as Soul Reaver's passive to also deal more damage.