Luckily, Frenetic Sliver can potentially protect your slivers from both
No, you can't buy singles on Arena, but the closest method is to buy a wildcard bundle, then craft away

1 Wildcard of any rarity from among the following: A common (12.5%), uncommon (58.3%), rare (15.6%), or mythic rare (2.6%) from Tarkir: Dragonstorm 's main set A common (4.6%) or uncommon (3.8%) showcase draconic frame card A rare (0.2%) or mythic rare (0.1%) showcase draconic frame card A rare (1.3%) or mythic rare (0.2%) borderless clan card A rare (0.6%) from among the borderless Sagas, Sieges, and lands or a borderless mythic rare Elspeth, Storm Slayer (less than 0.1%) A rare (0.2%) or mythic rare (less than 0.1%) borderless reversible dragon 1 Rare or mythic rare card from among the following: A rare (75%) or mythic rare (12.5%) from Tarkir: Dragonstorm 's main set A rare (0.8%) or mythic rare (0.6%) showcase draconic frame card A rare (6.4%) or mythic rare (1.2%) borderless clan card A rare (2.7%) from among the borderless Sagas, Sieges, and lands or a borderless mythic rare Elspeth, Storm Slayer (0.1%) A rare (0.8%) or mythic rare (0.9%) borderless reversible dragon 1 Traditional foil card of any rarity from among the following: A common (56.5%), uncommon (32%), rare (6.4%), or mythic rare (1.1%) from Tarkir: Dragonstorm 's main set A common (1.6%), uncommon (1.4%), rare (less than 0.1%), or mythic rare (less than 0.1%) showcase draconic frame card A rare (0.5%) or mythic rare (0.1%) borderless clan card A rare (0.2%) from among the borderless Sagas, Sieges, and lands or a borderless mythic rare Elspeth, Storm Slayer (less than 0.1%) A rare (0.1%) or mythic rare (less than 0.1%) borderless reversible dragon 1 Land card from among the following: A non-foil default frame basic land (7.0%) or full-art dragon's presence basic land (3.5%) A traditional foil default frame basic land (1.7%) or full-art dragon's presence basic land (0.9%) Can be 1 of 10 common two-color lands in non-foil (70.0%) or traditional foil (17.4%) 1 Art card or non-foil double-sided token Please note that Play Boosters no longer contain ad cards

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Youre probably going to take one or two of them and make token copies to get some real value
While Dredge isnt the blazing fast deck it used to be, having eight copies of Cathartic Reunion means that you can reliably put almost your entire deck in the graveyard on turn three or four