Note: This write-up is based on observations, not data or code.

Stop once a move is chosen:
1. Randomly select a target
2. Dismiss useless moves (e.g., Sunny Day if already sunny, Hypnosis if target is asleep)
3. Are any moves besides Solarbeam guaranteed to kill the target? If so, randomly select one of the moves
4. Are any moves besides Solarbeam a damage range to kill the target? If so, 50% to randomly select one of the moves
5. Is Solarbeam guaranteed to kill the target? If so, select Solarbeam
6. Is Solarbeam a damage range to kill the target? If so, 50% to select Solarbeam
7. Dismiss all attacking moves except the most-damaging move
8. Randomly select a move

Random = equal chance for all outcomes