Upper Attack is greatly exaggerated in writing about HMS. There's a lot more Upward Smash, which from everything I've tried is both more prominent AND more useful (mostly because of the dual spin nature of HMS and the fact most if not all of the HMS Attack AR's are significantly better in one direction vs the other), and this is something that wasn't really recognised back then.
However, in the case of Flat Core, given its low friction/difficult-to-control nature and the recoil from metal-on-metal contact, I strongly agree with Ga'Hooleone (based on having actually tried these exact setups today in fact!) that Circle Heavy is a much better choice for that setup (which is probably the best thing you can make from that). I'm a big proponent of 15g Circle Wide on Grip Flat Core and especially Grip Flat Core (Ultimate Mode), but this is not a case where that is a good idea. The fact Wolborg MS is not a particularly light HMS beyblade is a big part of this, of course.
@Ga'Hooleone: You sure on Sharp Core having better stamina than Semi-Flat Core? SFC has a really narrow tip diameter and better stability so theoretically at least it should still be a better option. Have you tried this out thoroughly yourself? It's been bugging me for a while so I might give it a shot tomorrow and just 1v1 them with equal-weight circle wides and Advance Balancers on both (or maybe just Metal Saucers given the circumstances this applies to and the fact it's not really different in either spin direction either), but if you've tried it thoroughly then there's little point me doing so again.
Would've done it ages ago but I only recently got a Semi Flat Core that isn't chipped or worn, and since then I've had more interesting HMS things to try or a tonne of non-HMS things to do, so I haven't got around to it. Also be interesting to see how each does against a BC2 defense combo (seeing as they can KO stuff if used right, and that could mean that even if SFC has lower stamina it might be a more viable choice in many situations).