Sonic has no priority over Snake's attacks. Shield dropping grenades stops Sonic's rolling attacks. Treat Sonic like Meta Knight when fighting with him. But, you can do more attacks against Sonic because of the priority of his attacks. Air dodge and evade a lot. Use your tilts after you place grenades and other explosives throughout the map. If you can't place C4 or the mine, just place grenades. Either way, Sonic will be crippled by grenades so that you can use your tilts. Oh yeah...it's fairly easy to grab Sonic out of his attacks. Just get used to Sonic's timing.