One fundamental evaluation criteria of an AI technique is its performance in the worst-case. For static strategies in extensive games, this can be computed using a best response computation. Conventionally, this requires a full game tree traversal. For very large games, such as poker, that traversal is infeasible to perform on modern hardware. In this paper, we detail a general technique for best response computations that can often avoid a full game tree traversal. Additionally, our method is specifically well-suited for parallel environments. We apply this approach to computing the worst-case performance of a number of strategies in heads-up limit Texas hold'em, which, prior to this work, was not possible. We explore these results thoroughly as they provide insight into the effects of abstraction on worst-case performance in large imperfect information games. This is a topic that has received much attention, but could not previously be examined outside of toy domains.