The irony of your situation is that, if you had one more creature that could attack Maeve, then you could win the game with Foul-Tongue Shriek. But you can only produce a maximum of six attacking creatures, and none of them are going to be able to pierce your opponent’s defenses for the final points of damage. You’re just going to have to clear a path.

1. Tap four islands to add (three used as generic mana) and four swamps to add (all used as generic mana) to your mana pool.
2. Spend to cast Ancestral Statue. When Ancestral Statue enters the battlefield, it gets +2/+2 until end of turn because of your Ambuscade Shaman’s ability. Ancestral Statue is now a 5/6 creature.
3. When Ancestral Statue enters the battlefield, its ability triggers. Choose to return your Profaner of the Dead to your hand.
4. Spend to cast Profaner of the Dead. When Profaner of the Dead enters the battlefield, it gets +2/+2 until end of turn because of your Ambuscade Shaman’s ability. Profaner of the Dead is now a 5/5 creature.
5. When Profaner of the Dead enters the battlefield, its Exploit ability triggers. Choose to sacrifice your Ancestral Statue; this returns all creatures Maeve controls with toughness less than 6 to her hand. This leaves Dragonlord Atarka as Maeve’s sole creature.
6. Declare the start of your combat phase, assigning your Silumgar’s Faithful, Hand of Silumgar, Dutiful Attendant, and Ambuscade Shaman to attack.
7. Tap your Dromoka Monument to add to your mana pool.
8. Spend to cast Foul-Tongue Shriek, targeting Maeve. Maeve loses 4 life (since you have four attacking creatures), and you gain 4 life. Maeve is now at 3 life. You are now at 16 life.
9. Maeve can block any one of your attacking creatures with her Dragonlord Atarka, but all of your other attackers will go unblocked. Maeve’s best choice would be to block either your Hand of Silumgar or your Ambuscade Shaman, but this will result in your other attacking creatures dealing a total of 3 damage to her, defeating Maeve.

UPDATE: Anastase points out a perfectly valid (and more appropriate) solution:

1. Declare the start of your combat phase, assigning your Sidisi’s Faithful, Hand of Silumgar, Dutiful Attendant, Ambuscade Shaman, and Profaner of the Dead to attack.
2. Tap a swamp to add to your mana pool.
3. Spend to cast Foul-Tongue Shriek, targeting Maeve. Maeve loses 5 life (since you have five attacking creatures), and you gain 5 life. Maeve is now at 2 life. You are now at 17 life.
4. Maeve may block any or all of your creatures, but your next steps depend on whether or not Maeve blocks your Dutiful Attendant.
5. If Maeve blocks your Dutiful Attendant:
   • Dutiful Attendant will die against any of Maeve’s creatures that block it.
   • When Dutiful Attendant dies, its ability triggers. Choose to return Qarsi Sadist from your graveyard to your hand.
   • End your combat phase.
6. If Maeve chooses not to block your Dutiful Attendant (and presumably blocks all of your other attacking creatures):
   • Dutiful Attendant deals 1 damage to Maeve. Maeve is now at 1 life.
   • End your combat phase.
   • At the beginning of your second main phase, tap two islands to add (all used as generic mana) and two swamps to add  to your mana pool.
   • Spend to cast Foul-Tongue Invocation on yourself. Choose to sacrifice your Dutiful Attendant.
   • When Dutiful Attendant dies, its ability triggers. Choose to return Qarsi Sadist from your graveyard to your hand.
7. During your second main phase, spend to cast Qarsi Sadist.
8. When Qarsi Sadist enters the battlefield, its Exploit ability triggers. Choose to sacrifice Qarsi Sadist to its own ability. This causes Maeve to lose 2 life (and you to gain 2 life), defeating her.