The bloodbraid elf one was pretty easy when I made an oath of druids / eureka deck. There's probably a few other ways to do it, like using cards that are cheaper than their listed casting cost, things like
Tombstalker,
Force of Will,
Stratadon, Bringer of the ____ Dawn, and so on, for the bigger numbers. Oath should make it simple enough, though. The other piece of advice I have for this one is that, in addition to your 4x10=40 mandatory cards, add more than 20 land. This is one of the few times where a deck greater than 60 cards is helpful, because 20/60 lands is insufficient, especially given the horrible mana curve in this challenge. I did 24 land in a 64 card deck, but that's because all I ever cast were
Enlightened Tutor and
Oath of Druids. If you use a non-Oath strategy, you probably need 2-4 more lands than that.
5:1 is just mean. That one took me a long time. There's only really one deck you can play, too, and that's one with a lot of Ring of Ma'Ruf and Wishes and then stuff to control the game with, like
Moat,
The Abyss,
Balance, etc. Even then, be prepared to play dozens of games, and play very very carefully, especially on the one where the 5:1 count is for removed from game cards. It is very easy to accidentally screw yourself by wishing twice, thus having 2 RFG cards before AI has 5. (I did it more than once

) I suggest Ring of Ma'Ruf on
Jester's Cap, recur it once, and then do whatever you need to for the win. (That, or wish for/maindeck
Path to Exile or
Swords to Plowshares in a creature deck matchup)
The 5:1 challenge you won't need as many wishes as I used (I had to get 10 Strip Mines, not 5) and the
Strip Mine one will be far easier with
Donate around. (I used Power Struggle at the time...

) You'll be able to fit more control/removal cards in their places, so it should be slightly easier, but still brutally hard.
The decklists I used for these two are somewhere here in the Decks and Strategy category.