I've been doing my own protests for 16 years now and if you're going into the formal hearing with only "comps" as your evidence, you'll lose. From the board's perspective, it's like being your own witness against a traffic ticket for rolling a stop sign. If your only defense is "I didn't do it", you'll lose. Yes, I understand "comps" are evidence, but the point is you are completely biased because the selected comps lower your personal taxes vs. the appraiser's comps don't affect him one bit. He may be biased, but you're 100x more.
First, your biggest hope of getting a reduction is in the informal meeting. The appraiser is much more willing to look at your comps and see it from your side. In the formal meeting, they're your opponent that you have to overcome. But in the informal, there's no jury that each of you have to convince. Just two people trying to come to a resolution together. Sometimes you don't get a flexible appraiser, but sometimes you do. I protested 6 properties this year and the appraiser accepted my opinion's value on 4 of them.
If you do go to the formal (try to avoid at all cost), bring evidence that the appraiser didn't know about. Cracked foundation, hail damaged roof, etc. Something that shows the board that the appraiser simply didn't have all the information when doing his appraisal. Bring pictures and repair estimates.
Look at their evidence pack before you go and do research on each of their comparable properties. Look for anything that can justify a different adjustment than they used. Better location, nicer curbs appeal, etc. Take pictures of your ****ty house vs. the comparables.
But in the end, you'll lose in the formal 9 times out of 10. A homeowner with a vested interest in the outcome vs. a somewhat unbiased professional appraiser. Again, it's like a driver being his own witness vs. the cop that wrote you the ticket. Unless you get a sympathetic jury, you're going to lose. If the DA gives you any sort of deal before the trial, take it.