With warded Battlements in play, you can attack with a base 3-power creature alongside dowsing tyrannodon. Steps are 1:

1. Have Dowsing Tyrannodon, a 3-power creature, and warded battlements in play
2. in declare attackers step, click on the 3-power creature to declare it as an attacker
3. Actual: warded battlements sets that creature's power to 4, allowing you to attack with the tyrannodon
4. Expected: the creature's power doesn't change until attackers are actually declared and tyrannodon can't attack