Hi,
Metal ( lev 10) - first I tried with Eagle ( lev9) bbb -lost, 2nd was with Blue Moon( lev7) bmb - lost, and on my 3rd round I used Firestorm ( lev 8) bbm - and we beat Metal !!!
Maybe this will help u. I think most of dragons can get him(Metal) , when u try just see which one more close to win and remember what technique did u use and next time try this dragon again with different technique ( bbb or bmb or mbb.....)