Walid made a good play by copying your Deathpact Angel, but you can make a great play by copying the creature that’s copying your Deathpact Angel. Your opponent may be planning to use Pandemonium to his own benefit, but in this case, it might just as easily hasten his demise.

1. Before Walid’s turn ends, tap a plains to add (used as generic mana) and Watery Grave to add  to your mana pool.
2. Spend to cast Refocus on your Artisan of Forms.
3. Artisan of Forms’ ability triggers. Choose to have it become a copy of Walid’s Progenitor Mimic. Because the Mimic is currently a copy of Deathpact Angel (with the added token-producing ability), Artisan of Forms becomes a copy of Deathpact Angel with the token-producing ability.
4. Refocus allows you to draw a card. Draw Tidal Bore.
5. Allow Walid’s turn to end. Your turn begins.
6. At the beginning of your upkeep, the abilities of your Artisan of Forms/Deathpact Angel and Demonic Taskmaster both trigger. Choose to have the Artisan’s ability resolve, then the Taskmaster’s.
7. Your Artisan of Forms’/Deathpact Angel’s ability resolves first, putting a Deathpact Angel token onto the battlefield under your control.
8. Pandemonium’s ability triggers. Choose to have your Deathpact Angel token deal damage equal to its power to Walid’s Deathpact Angel token, destroying it.
9. When Walid’s Deathpact Angel token dies, its ability triggers. Walid puts a 1/1 Cleric token onto the battlefield under his control.
10. Pandemonium’s ability triggers for Walid. Walid may choose to have the Cleric token deal 1 damage to you or any one of your creatures. If Walid chooses to deal 1 damage to your Sootstoke Kindler, tap your Field Surgeon to prevent the damage.
11. Your Demonic Taskmaster’s ability triggers. Choose to sacrifice your Deathpact Angel token.
12. When your Deathpact Angel token dies, its ability triggers, putting a 1/1 Cleric token onto the battlefield under your control.
13. Pandemonium’s ability triggers. Choose to have the Cleric token deal 1 damage to Walid. Walid is now at 19 life.
14. During your draw step, draw a Plains.
15. At the beginning of your first main phase, play the Plains.
16. Tap Sootstoke Kindler to give your Cleric token haste until end of turn. The Cleric token is a white and black creature, so it can be the target of the Kindler’s ability.
17. Tap two plains for (one used as generic mana), and tap three swamps and Watery Grave for (two used as generic mana).
18. Spend and tap and sacrifice your Cleric token to return your Deathpact Angel card from your graveyard to the battlefield.
19. Pandemonium’s ability triggers. Choose to have your Deathpact Angel card deal 5 damage to Walid. Walid is now at 14 life.
20. Return Watery Grave to your hand in order to cast Tidal Bore for its alternate cost. Choose to untap Sootstoke Kindler.
21. Tap Sootstoke Kindler to give your Deathpact Angel card haste until end of turn. The Angel is a white and black creature, so it can be the target of the Kindler’s ability.
22. Declare the start of your combat phase, assigning your Artisan of Forms (copying Deathpact Angel), Demonic Taskmaster, and Deathpact Angel (card) to attack.
23. Walid’s lone untapped creature, the Cleric token, is unable to block any of your flyers. Your creatures deal a total of 14 damage to Walid, defeating him.

Update (22/04/2015): TBlade2001 posits an alternative solution:

1. Before Walid’s turn ends, tap a plains to add (used as generic mana) and Watery Grave to add  to your mana pool.
2. Spend to cast Refocus on your Artisan of Forms.
3. Artisan of Forms’ ability triggers. Choose to have it become a copy of Walid’s Progenitor Mimic. Because the Mimic is currently a copy of Deathpact Angel (with the added token-producing ability), Artisan of Forms becomes a copy of Deathpact Angel with the token-producing ability.
4. Refocus allows you to draw a card. Draw Tidal Bore.
5. Allow Walid’s turn to end. Your turn begins.
6. At the beginning of your upkeep, the abilities of your Artisan of Forms/Deathpact Angel and Demonic Taskmaster both trigger. Choose to have the Artisan’s ability resolve, then the Taskmaster’s.
7. Your Artisan of Forms’/Deathpact Angel’s ability resolves first, putting a Deathpact Angel token onto the battlefield under your control.
8. Pandemonium’s ability triggers. Choose to have your Deathpact Angel token deal damage equal to its power to Walid. Walid is now at 15 life.
9. Your Demonic Taskmaster’s ability triggers. In response, tap your Sootstoke Kindler to give your Deathpact Angel token haste until end of turn.
10. Your Demonic Taskmaster’s ability resolves. Choose to sacrifice your Sootstoke Kindler.
11. During your draw step, draw a Plains.
12. At the beginning of your first main phase, play the Plains.
14. Tap a plains to add (one used as generic mana), and tap Watery Grave to add to your mana pool.
15. Spend  to cast Tidal Bore, tapping Walid’s Deathpact Angel token.
16. Declare the start of your combat phase, assigning your Artisan of Forms (copying Deathpact Angel), Demonic Taskmaster, Field Surgeon, and Deathpact Angel (card) to attack.
17. Walid’s has no untapped creatures and is unable to block any of your attackers. Your creatures deal a total of 15 damage to Walid, defeating him.