A 5/4 flying creature won’t make that much of a difference against 15 life, so the Butcher of the Horde only forms part of the equation here. The rest comes from a more unlikely source: your Kheru Bloodsucker. You just have to find a way to get a bunch of creatures with four or more toughness on the battlefield…
- Tap three plains to add
, two mountains to add 
, and three swamps and Scoured Barrens to add 
(two used as generic mana) to your mana pool. - Spend to cast Mardu Ascendancy.
- Spend
to cast Butcher of the Horde. - Spend to cast Feat of Resistance on your Sultai Scavenger, putting a +1/+1 counter on it and giving it protection from the color of your choice until end of turn. (The color does not matter.) Your Sultai Scavenger is now a 4/4 creature.
- Activate the Butcher of the Horde’s ability, sacrificing your Sultai Scavenger. Choose to give Butcher of the Horde haste until end of turn.
- Kheru Bloodsucker’s ability triggers, forcing Margaret to lose 2 life and you to gain 2 life. Margaret is now at 13 life. You are now at 4 life.
- Declare the start of your combat phase, assigning your Kheru Bloodsucker and your Butcher of the Horde to attack. Mardu Ascendancy triggers, and you put two 1/1 red Goblin creature tokens onto the battlefield tapped and attacking.
- Sacrifice Mardu Ascendancy to give all creatures you control +0/+3 until end of turn. Your Butcher of the Horde is now a 5/7 creature, your Kheru Bloodsucker is now a 2/5 creature, and each of the two goblin tokens is now a 1/4 creature.
- Margaret may choose to block a creature with her untapped Mystic of the Hidden Way. The largest attacking creature she can block is your Kheru Bloodsucker, in which case the Mystic dies in combat and your creatures deal a total of 7 damage to her (from the Butcher of the Horde and the two goblin tokens). Margaret is now at 6 life.
- At the beginning of your second main phase, activate Butcher of the Horde’s ability, sacrificing a goblin token. (It does not matter what ability you choose to give Butcher of the Horde.)
- Kheru Bloodsucker’s ability triggers, forcing Margaret to lose 2 life and you to gain 2 life. Margaret is now at 4 life. You are now at 6 life.
- Activate Butcher of the Horde’s ability again, sacrificing your second goblin token. (It does not matter what ability you choose to give Butcher of the Horde.)
- Kheru Bloodsucker’s ability triggers, forcing Margaret to lose 2 life and you to gain 2 life. Margaret is now at 2 life. You are now at 8 life.
- Activate Butcher of the Horde’s ability again, sacrificing Kheru Bloodsucker. (It does not matter what ability you choose to give Butcher of the Horde.)
- Kheru Bloodsucker’s ability triggers, forcing Margaret to lose 2 life and you to gain 2 life. Margaret is now at 0 life and is defeated.