Do abilities interrupt transform effects?
Yes, your opponent will be able to kill your Ulrich.

Whenever a spell is cast, or an ability is activated, it goes on the top of the stack. When both players pass priority, the top of the stack resolves. In this case, your opponent will be able to respond to your Waxing Moon spell by activating Scorpion God, and the -1/-1 counters will be placed on Ulrich, killing him before Waxing Moon ever resolves.

The stack will look like this at each step:

Step 1; opponent activates Scorpion God's ability 4 times, targeting Ulrich:


Ulrich gets a -1/-1 counter
Ulrich gets a -1/-1 counter
Ulrich gets a -1/-1 counter
Ulrich gets a -1/-1 counter


Step 2; you respond with Waxing Moon:


Waxing Moon
Ulrich gets a -1/-1 counter
Ulrich gets a -1/-1 counter
Ulrich gets a -1/-1 counter
Ulrich gets a -1/-1 counter


Step 3; your opponent responds by activating Scorpion God's ability 4 times:


Ulrich gets a -1/-1 counter
Ulrich gets a -1/-1 counter
Ulrich gets a -1/-1 counter
Ulrich gets a -1/-1 counter
Waxing Moon
Ulrich gets a -1/-1 counter
Ulrich gets a -1/-1 counter
Ulrich gets a -1/-1 counter
Ulrich gets a -1/-1 counter


Then, if neither of you do anything else, the stack will resolve from top to bottom. 4 steps down, Ulrich will go to the graveyard for having 0 toughness. Then Waxing Moon will be countered because it has no legal target (Ulrich is in the graveyard now). Then each of the 4 original activated abilities will be countered because they have no legal target.