If you're creating a video that contains content that belongs to you without permission there is no sure fire way to prevent yourself from receiving a copyright claim against you. While reaction videos could be argued as a review to a product (in this case, a review to the video you're reacting to) which falls under the guise of fair use, it is important to note that fair use is not a legal right, but rather a legal defense. Essentially, the only way to prevent your videos from being flagged is to request permission from the creator (which they can decline, even if you believe using the content would be considered fair use.)
Essentially, there is no legal loophole to prevent claims, you have to fight any claims against your video if you wish to upload using content that belongs to someone else. Simple as that.