Defense Grid is definitely the best card in the deck to bypass counterspells. Otherwise, depending on your splash color, cards like Abeyance and City of solitude can do the trick as well.
Beware that people tend to bring in artifact hate post board though.
Toxic Deluge can be too costly to be effective sometimes but is still better than most alternatives (such as Pyroclasm, Bountu's last reckoning, Virtue's Ruin, ...).
Balance is godly if you are splashing white, and as Soly said Karakas is a must have since it deals with Thalia and Lavinia.
Engineered Explosives and Echoing Truth are also worth considering as catch-all answers.
Pack Rat used to be a great plan B vs hate decks but Ballista slammed that door shut for good. Make sure to have Mentor and/or Tinker Bot as a more resilient win-con in the 75 though.
Oath of Druids can be an option too with a green splash.
Welcome to MTGO! Glad to have you.
- In most cases you want to leave the Traps in the SB against Xerox and just lean on the discard, Flusterstorms and Defense Grids. The problem with Traps is that you game plan is basically trying to limit the amount of interaction the opponent has access to during your turn between the discard and Defense Grids - your opponent can't cast multiple spells if you have a Grid in play and likely won't have multiple counters if you've been stripping their hand - so Trap is accomplishing the opposite of what you want it to do. Board it in against PO and the Mirror. I also like it against Shops because after you Hurkyl's them, they will generally play out their threats first and then let you trap whatever their prison piece is. That depends on the exact list and SB configuration though.
- If you really want the sweeper effect, Deluge is probably your best bet. However, SB space is limited. In general, the dealbreaker on Toxic Deluge is the lack of utility of the card. Karakas is an extra mana source against Shops and a discard spell against Control/Combo decks. Fatal Push also has some function as removal against Shops (hitting Revoker) though I've honestly cut it for another Brutality. The problem with Deluge is that it's only really good at dealing with these hatebear/eldrazi matchups and that in my opinion isn't enough to justify the SB slot. Now, your experience may vary. I'm a nerd so I keep track of my matchups and I see that over the past month I played against Eldrazi 4.5% of the time, "Fish" 6.3% of the time, and a couple Humans decks (like <2%). Conversely, Xerox + Survival has been ~35% of my matchups, with Shops another 10%. Having Karakas and Brutality are then more valuable because they do more in those matchups.
I like Chains of Mephistopheles against PO, since they usually don’t have enchantment removal against DPS. Chains shuts down much of their deck. You may need Karakas or Liliana against BW PO because they bring in Kambal.
Against Thalia, I like creatures. Phyrexian Obliterator, for his immunity to Ballista, as the finisher, and a collection of variable underlings such as Pack Rat, Black Knight, and Plague Spitter. This usually catches them off guard.
I haven’t played against the new card , Lavinia, but I would probably cut Dark Petition and go back to Burning Wish.
I like the chains idea, but can Ritual Storm really afford to have sb slots dedicated to the outcome matchup?
I love Phyrexian Obliterator but since we'd be bringing it in mana denial/sphere matchups, can we reliably pay XBBBB to cast that thing when Ritual is mostly a colored mana fix?
Black knight just isn't good enough and plague spitter doesn't deal with Lavinia without therapy.
Pack rat just doesn't pan out against shops between revokers and Ballistas.
I think the only reliable plan B in the current state of things is Tinker Bot and/or Mentor if you splash white.
I like the Burning wish idea, it's much better than Petition in the face of Lavinia and/or at finding answers to disruption. It also does a better job at abusing broken cards like LED, Demonic Consultation and Gifts Ungiven. But Dark Petition has a lot more raw power and consistency.
@stasis_kismet In my experience, bringing in Hurkyl's has not been worth it because being reactive against PO tends to be a losing plan since they plenty of countermagic. I have found the PO matchup extremely difficult. Grid + discard has been the best plan I have found, but even that is typically not enough. They're just better able to disrupt your gameplan than you're able to disrupt theirs, and they're just about as fast as you are.
If you want to beat outcome, you can sideboard Sadistic Sacrament. This card will beat the pants off most outcome lists.
I always wonder why Cap effects aren't played more often, when there are so many decklists out there that have all of 1-3 win conditions.