Yeah, I honestly don't think Sylvia is that hard of a match in general; Aegis is certainly tricky but has some weaknesses and otherwise she doesn't do anything too unfair - her deck just has a bunch of fairly durable creatures but they're offensively weak - if you can deal with her spells and Mirror Force (which she uses way too early if you don't have anything tough out yet), you should be able to win most of the time with even mediocre decks. Even before I added Sacred Tree, I often just beat her with my early version Water-deck before she got Aegis down (as long as she never starts the turn with two creatures and you destroy her magic cards, she basically can't get it out) - that's how I got the initial victory over her. I definitely find Sacred Tree worth a lot vs. both, her and Luca (and also most other high-end opponents).
I think Gemini, some draws of Eilaira's deck (hers often does nothing and sometimes just totally crushes you), and maybe Cammy are the hardest of the bunch (Cammy has Fever Time to fix her rolls and the ability to block your spellcards, which is really strong against most decent decks; she has something). Luca is occasionally pretty tough too; her ability to block attacks entirely without permanent magic cards is frustrating since there are few enough ways to deal with it (of course, the constant QTEs are the biggest pain in that match).
I actually just got around to collecting all the Freeze-cards and was thinking of fiddling around with it (and a Ninja-deck); I'd definitely be interested in the shell you're working with.
I also tested
Illyaster 's Triangle Thor-deck, which is certainly good at smacking around Sylvia (I don't think I ever lost that match even with terrible draws), though I did find that it's somewhat vulnerable to flooding out on non-Light cards for Reservoir (I found that I usually got the first Thor out with Reservoir) and doesn't deal with Luca's deck or traps all that well (the "Destroy trap"-card from the first booster looks interesting). I cut it down to 21 cards cutting a Bookmaker, Burning Down & Magician alongside single Replenishments [I basically never wanna use Replenishments to Triangle, and Cool Time makes two-three awkward until you're already cruising] to try and counteract the non-Light flood (I was uneasy about using Magician if I had Reservoir out simply because I didn't feel I could count on drawing two light-cards). I also noticed it occasionally has a problem with running out of Thors in the deck for Triangle before you get to reshuffle if you draw multiples early; cutting down the decksize seems to help with that too. I did try Lightning Magician, which is a fun card through it's not a Warrior for Thor, so probably not optimal - but after you're done chaining Thors it gives you a lot of extra value for the Lightnings you're left with and it's good at stalling early (an interesting feature of the deck is that most of your cards are actually kinda useless most of the time; you really don't care about the Holy Knight or the Young Hero for anything but tributing, discarding, or sacrifing - so you often have discard fodder).
Good luck! I'm sure there are plenty of other decent decks, but that one stood out to me for how early and easily you can build it, how consistent and powerful it is, and how easy it is to insert silver bullet -like answers to specific issues (since the deck draws a lot and cycles through a ton of cards, you get to see your whole deck a bunch of times over a game; you can often go through the deck in 2 turns when you need a specific answer). None of the other shells have impressed me quite as much (and most require more SR/UR cards as workhorses, while a single Poseidon is all you need for a Water-deck to be amazing).
