It occurs to me that I neglected to included suggestions for items for Ambrosia in my earlier write-up of her as I did with Higashi. Allow me to overcompensate.
I honestly had a bit in there about it or at least about my outlined intent to talk about it but it got lost in the wash somehow. The reason is likely that there's no real clear-cut winner. Unlike Higashi you can't just slot General's and call it a day. No one item really stands out as the single best option. No counter-example floats out of the murky alternatives to arise as the indie selection for non-conformists. Instead, you've got several good options and a bunch of not so good options. Let's run through them altogether.
Now, Ambrosia is a Vampire, what I refer to as a Necro because I'm weird like that, and, as such, her card has a black bordered edge. What this means is that she can make use of items with a similar border so those are a logical place to start. There are five of them and although they're rather underwhelming as a group they do offer some interesting possibilities.
Beyond those five black item cards, Ambrosia - like every other card - can use any item with a neutral gray colored edge. I refer to these as general-use items and there are six of them. Some solid options here as well along with some that are less so.
- Blood Vial: The best Necro item, hands down. Damaged cards that are in your hand will slowly recover their health. And what this does is to set your resting health gain to 3 instead of the normal 1. If you like to toss Ambrosia out there and have her bang it out and soak up a lot of damage before taking her off the board this lets her rest up and get back into the fighting stronger or, at least, sooner. She does have Vamp Kiss, though, which she can use to keep her health up as well, so it's not as crucial for her. And, in any event, you need to get through the gate of switching out a damaged character to gain any benefit at all - and that's not always easy.
- Gem of Souls: Personally, I love the Gem of Souls since it offers so much raw power. Adding innate abilities onto your card can turn them into a juggernaut. But it's hard to get working reliably. You need to get kills and you need to have a certain card making them and then that card needs to stay alive long enough to make use of their new abilities - none of which are easy so it's usually best to just forget about this item even though it's so cool. But if there's any card that can make good use of the Soul Gem, it's a card as lethal as Ambrosia is. She's a killer who can take apart her opponents and this lets you benefit even more from that.
- Necronomic Tomes: Adds a chance to proc a small life drain on each dark magic skill you cast. Thorny is loaded down with dark magic attacks so this makes them all better. It works a little like General's in that it gives you a boost to your damage. A 30% chance to take another 10 off your opponent's life bar isn't as consistent and it's not as damaging, either. It works out to +3 to each of your attacks on average. But that's over the long term, in the short term you're as likely to get an extra 60 damage in 10 attacks as you are to get nothing at all. But not only do you add 3 damage your opponent gets -3. It's a life drain so it's not just raw damage it's a swing in health (assuming you've lost some) that can really help you out. More health in your bar lets you last longer, after all so it's a solid choice for Ambrosia.
- Ring of Curses: My problem with Ring of Curses is that you have to take hits before it matters and it takes an awful lot of hits before it actually does. Ambrosia can recover a lot of that damage but I still think there are better items to slot.
- Phylactery: Like the Gem of Souls this is another all or nothing card that's not going to matter until it does in which case it's going to make you extremely happy. When it procs it keeps you from being killed and that gives you another combat round to make a difference. It's a bit too inconsistent for me but if you like to gamble on the long odds, it can work wonders. It might not be best with Ambrosia, though, because her attacks are generally low priority and, generally, cost a lot. Lasting another round doesn't matter if you can't do anything with it. And that's likely to happen if your opponent can hit you before you can hit them or you don't have any energy left (I mean, sure, you can swap out but they know that's all you can do and while you can rest and outguess them, you really have to guess right or your once in a blue moon proc is wasted.).
- General's Insignia: A really strong item that can help out a lot. General's will add 6 damage to Vamp Kiss and that damage works overtime because it's stolen and added to your health bar - instead of a 25 hit for a 50 point swing, you get a 31 hit for a 62 health swing. And that extra damage turns Spiked Boots from chip damage into an actual threat. But it doesn't do anything for Spectral Choke or Bleeding Ritual and, so, is a bit of a waste.
- Girdle of Iron Will: Somewhat of an overlooked choice and maybe not the best since it's not Higashi-proofed (Open Palm causes Paralysis, not Stun and while the two are almost exactly the same the difference is that one is covered by the Girdle and one isn't. Guess which?). But Thorn has a lot of low priority attacks. This card gives you a lot of assurance that you won't get zonked by Helene's Shield Bash or Popo's Knee Bash or Sweep or anything else that could keep them from going off. Nothing like Vamping through a Bash and then finishing them off with a Spectral while they're still gasping in disbelief.
- Healing Salve: I'm still convinced this is a bad card - just doesn't heal enough fast enough to be important and where it shines are the gameplay states I think should be avoided. But if you play Ambrosia in Drain Tank mode - using Vamp Kiss to soak the damage you're taking as you whittle someone down and bleed them off - then this card can help you do that by extending your health bar even further. I wouldn't do it but you might.
- Mindreader's Chalice: Generally, you can't go wrong with the Pimp Cup. It intimidates your opponent into staying on the field and punishes them if you catch them leaving. But Ambrosia is already really hard on switching cards. Spectral hits fleeing so they can't run from it. Bleeding Ritual continues to tick over even if they leave the field so they're less likely to withdraw and more likely to stick it out as they go down in a blaze of glory. I can see using it but I think it's suboptimal.
- Stoneheel Totem: Stoneheel works best on cards that work best at one range. Becase those are the cards opponents are going to be very likely to shift ranges away from. And while that 5 energy might not look like a lot and probably won't stop them from running away from a big bomb like Spectral, it adds up and wrecks their energy curves over time, really disabling them. The current Ambrosia features Bleeding Ritual, though, which reduces her need to stay at close-range by a lot so a Stoneheel isn't as nice on her as it is on, say, Ashi, but it can work.
- Yellow Rock of Cowards: Again, Spectral already hammers anyone leaving a lot worse than the threat of 5 damage will. And Bleeding Ritual will continue to bleed them if they get away making them more likely to stay. What it's good for is causing some chip damage for those times when your opponent is going to run from a bad match-up and you're not going to intercept - like, say, you swap in Ambrosia and they decide to get out of there while the getting's good and they have enough health/armor to absorb a Spectral - because, there, it kicks them in the shins just that little bit more. But since that card will regenerate while in your opponent's hand, it's not really threatening and I think there's better.
Like I said, a lot of good options but nothing that I could point to as clearly heads above the rest. If I had to pick one, though, I'd go with either the Vial or Tome and, at the moment, I'd edge towards Tomes. Really, though, it depends on your playstyle and what, exactly, you want to get out of your item selection.