Sinuous Striker query around pump in reaction to combat/defending
Sinuous Striker will die without dealing combat damage.  Your opponent's creature will remain blocked, but will not be destroyed.



The problem with this combo is that there is no way to activate Sinuous Striker's ability without state-based actions being checked immediately afterwards.  State-based actions are checked immediately after any ability resolves.

Let's assume that the Declare Attackers Step is complete.  Here is the order of events:


You declare that your Sinuous Striker will block your opponent's creature.509.1
The active player (your opponent) gets priority.509.4
Your opponent passes, giving you priority.116.3d
You activate Sinuous Striker's ability twice.  You and your opponent both pass (twice) allowing both of those abilities to resolve.  Sinuous Striker is now a 4/0 creature.
The active player (your opponent) will get priority after the spell resolves,116.3b so a check for state-based actions is triggered.704.3
Sinuous Striker is destroyed by a state-based action.704.5f
Combat damage is assigned.510.1  Your opponent's creature remains blocked510.1c but takes no combat damage.