Possible counters
With the exception of ice rotom a dragon type can do wonders 4 defense dialga would be best since it also takes neutral damage from ice and takes no more than half from the rest, kingdra also resists ice but takes neutral damage from all others but fire; Hydregion, zekrom, like i said any dragon type works as long as you watch out for ice rotom. If you don't like dragons Rampardos is good since mold breaker will let you earthquake most forms and use rock moves on flying rotom. Blissy is also good since rotom has no physical attacks and it can absorb hits and waste rotoms pp for thunder, overheat etc.