A wallbreaker Pokemon is a Pokemon that has such a high attack or special attack high enough to render any protective wall, defensive boosting or trick moves useless. These moves refer to but are not limited to moves such as substitute, iron defence or trick. If you are able to boost the stats of these Pokemon or give them powerful moves that are virtually impossible to defend against, it is very tricky to put up walls to defend against them. Hence the term, wall breaker Pokemon.