Is it just me or are the roaming pokemon in gen V easy to catch? I looked up the bulletin boards in the gates and Thundurus was only on route 6, 7 and 8. I flew to Icirrus city then went to route 8 and went back. I repeated it until the weather was stormy on route 8, then I met a Thundurus weakened it, then in the next encounter I brought it to sleep and then after some encounters I catched it. It was much easier than in the other generations.