This is a tricky question, partly because it depends on each player's understanding of the exact steps of combat.
To try and simplify, there's several "priority windows" that open during combat, and each player has a chance to play things to affect combat. First, you propose combat: no characters are exhausted yet, but you say what you're attacking with and what you're attacking. This proposal effect is added to the chain, and when it resolves, if the attacking and defending characters are still legal, the proposed attacker exhausts, becomes an attacker, and begins combat.
After this, a priority window called the Attack Window opens. Again, both players can play powers or cards to affect combat. After this window closes, we reach the Protect Point, which is where your opponent can exhaust a character to protect and become the defender. This doesn't use the chain, and cannot be responded to until the character is exhausted. After this there is another priority window (the Defend Window), then combat damage is dealt, and the turn player's main priority window opens again.
Now, on to your specific example. Your Form of the Serpent will get a counter when the Attack Window opens, and you can remove the counters to kill a potential protector. However, if you elect to do nothing, your opponent can protect with an ally. If you kill that ally your hero is still attacking that ally and not the opposing hero. If you want to avoid your opponent protecting against your hero, you'll have to kill off Doshura before he protects.
However, keep in mind that your opponent can't "rush" to the protect point to keep you from killing his ally. If he says his ally is protecting immediately after you declare your attack, you can force him to "back up" so you can play more effects before reaching the protect point. If this is the case, your opponent can then declare a different ally as a protector. However, if you declare your attack and then ask your opponent if he's going to protect, or similarly display that you aren't going to use your ability's power, you have lost your opportunity to kill the opposing ally before he protects. You can still use Form of the Serpent to kill him before combat damage is dealt, but your hero will then deal no combat damage.
WoW characters:
Jadorel, 70 Night Elf Druid, 44/0/17 spec, Agamaggan server; raiding character.
Sethala, 55 Blood Elf Mage, 5/41/0 spec, Rivendare server; PvP character.