I have some stuff on what I consider one of Bowser's worst matchups, Zero Suit Samus.
First, why it's bad:
1: ZSS has one of the best neutral games in PM right now if the player is technical enough. She has a safe, dangerous projectile, high mobility on the ground, tons of long-reach zoning moves that lead into devastating combos, and a down-B that gives her so much safe air mobility and options it's almost unreal. If a good ZSS player decides to run away there is almost nothing you can do even on a small stage.
2. ZSS has dozens of options for comboing Bowser to oblivion. She can easily get 0-60 or more from simple dthrow into usmash and follow-up air combos, and if she manages to land a dair you can expect to get hit with at least six more even with good DI, followed by an uair or fair, depending on circumstances and how the ZSS feels like killing you.
3: Bowser has no safe recovery options against ZSS; a simple dsmash from the stage is next to impossible to avoid, requiring frame-perfect timing and a flawless sweet-spot. I've managed it maybe twice and my recovery skills with Bowser are supremely honed. If you get hit with the dsmash your only option is a stage-tech after the stun wears off, which is only possible if you are close enough to it, as is not always or even often the case; failing that, expect a fsmash or ftilt gimp, every time.
4: A skilled ZSS is probably the hardest tether character for Bowser to gimp. With her down-B options allowing for either a high or low approach and helping her save her jump, the second-longest tether range (I think), and plenty of low-lag airs to deter casual OtS approaches, it takes some perfect hard reads and timing to keep her from grabbing the edge, and if you go for a ledge-hog instead, your only really useful punish for her tether-hop is a back-throw from the Klaw, which requires both a good read on when she begins rising and perfect timing on the attack since she suffers almost no lag from her hop.
So yeah, all told I think this match is unfavorable. But no match is unwinnable. Here are the things I've learned fighting my expert ZSS friend:
1: Stay off the ground as much as possible while ZSS is within medium range. She has tons of options for zoning and punishing at long range on the ground and enough mobility to punish anything you do from about a Wario Ware length away, our more if you miss a cancel or do something stupid, and her punishes all hurt.
2. Don't approach in the air. I know that makes this match sound hopeless, but bear with me. ZSS has just as good options for punishing air approaches from the ground as she does punishing ground ones.
3: Don't chase ZSS. Just don't. You can't catch her. You can follow at enough distance to avoid sudden attacks or direction chances to keep up pressure, but don't ever try to attack while she is running.
4: Get perfect at Power Shielding her stun gun. It's one of the easier projectiles to PS and the rewards are great, leaving her open to a quick punish more often than not.
5: Don't get greedy with gimps. Ledge-hog and punish with a fair or nair then back off or, if possible, tech chase. Only go for the Klaw reversal if you think you're in your opponents head and he's high enough percent for it to matter.
6: Save your second jump as late as possible when recovering. It is far easier to sweet-spot from a jump than from Bowser's up-B. When it isn't possible to recover with just a jump, recover high, and when that isn't possible try to stall beside the ledge during the up-B to bait an early dsmash. None of that is safe or guaranteed, but it beats certain death if you just go straight for the ledge.
This matchup is one about patience. A good ZSS will try to bait you into getting aggressive, but it's always a trap. Play super-defensive even on small stages, rack up single hit punishes whenever possible, use the safest and most reliable options when punishing over the edge, don't pres your advantage too hard, and play the clock. Nothing makes this match easy, but this strategy will give Bowser his best shot in my experience.
