Find a reputable, experienced groomer. It’s not unusual for rapidash to be skittish around strangers, so your rapidash’s behavior shouldn’t be surprising for them. In fact, a good groomer should already have in-depth knowledge on how to keep even the most unruly or anxious rapidash calm throughout the entire process.
For extra reassurance, stand within view of your rapidash, preferably with your hands on his snout. Not only will this position keep him focused on you (and possibly calm, as his field of vision will be taken up by someone he trusts), but it will discourage fiery outbursts, as you will be in physical contact with your rapidash at all times. Don’t worry if you yourself get burned; unless your rapidash is particularly anxious, the likelihood of that happening is low.
Also, be prepared to offer positive reinforcement in the form of apple slices or carrots after the session. The more you reward your rapidash for being well-behaved (and, for that matter, the more you promise rewards for good behavior prior to your first session), the easier the transition will be.