Surtland Flinger’s triggered ability goes on the stack without a target. While that ability is resolving, you may sacrifice another creature. When you do, the reflexive triggered ability triggers and you pick a target to be dealt damage. This is different from effects that say “If you do” in that players may take actions after you’ve sacrificed the creature but before damage is dealt.

Use the power of the creature as it was sacrificed to determine how much damage is dealt. Damage will be dealt before blockers are declared.