The options:

• Flatten Quartermasters now
• Flatten Shorecrasher now
• Flatten Quartermasters in combat
• Flatten Shorecrasher in combat
• Do nothing

What happens:

• Flatten Quartermasters: counters go on it, becomes a 4/4. He swings with a 4/4 and a 3/3. You block the 4/4 with Hand of Silumgar, the 3/3 with Vulturus Aven. You have a 4/3 and your opponent has a 3/3 at end of turn. Next turn, Pyrotechnics removes his other creature and you swing for an unblocked 4. You have a regular 4/3 on the board.
  
• Flatten Shorecrashers: counters go on Quartermasters because it's his only creature. You have a Hand of Silumgar, so he passes turn. At the end of turn, you have a 4/3, a 2/1, and a 2/3 flyer and he has an 8/8. Next turn, Pyrotechnics, you attack with everything. After declaring blocks (he blocks the 4/3 because the other has flying) which causes your 4/3 and 8/8 to trade. Hits for 4, you have a 2/1 deathtouch and a 2/3 flyer on the board.
• Do nothing: He swings with a 6/6 and a 5/5. You block the 6/6 with your 2/1 deathtouch and the 5/5 with the 2/3 flyer. End of turn, he's got a 5/5 on the board, and you have a 4/3 on the board. On your turn, Pyrotechnics, then you swing with the 4/3, Pyrotechnics and his 5/5 dies, but he takes no damage. The board state is empty (because your 4/3 died, too). He's at 9, you're at 5. You're not doing this.
• Flatten Shorecrashers after the spell resolves: You take out Shorecrashers with your 2/3 which lives. Hand of Silumgar trades with Quartermasters. End of turn, you have a 2/3 flier and a 4/3 vanilla. Next turn, Pyrotechnics. You'll wind up with a 2/3 flyer on the board and he takes 2 damage.
• Flatten Quartermasters after the spell resolves: he swings with a 2/2 and a 5/5. You block the 2/2 with your 2/3 flyer and the 5/5 with your 2/1 deathtouch. Both his creatures die, leaving you with a 2/3 flyer and a 4/3 vanilla. You swing for 6 (4 + 2 in air) next turn, then Pyrotechnics for the win.

You flatten Quartermasters in combat and win next turn.