I'm battling cynthia and I need to use Iron tail FIRST so Ive given max carbos to my pikachu,will my pikachu attack first?
Well Ill say this you may be able to attack first but it won't finish him of in one blow the next turn Garchomp will finish you off with Earthquake. I would recommend using a Weavile or a good Ice type.
Level 63 Pikachu, 31 IVs, 252 EVs, Speed Increasing Nature - 195 Speed
Level 62 Garchomp, 31 IVs, 252 EVs, Speed Increasing Nature - 209 Speed
Garchomp would be faster according to here.