There is no chance to consider any team in the Premier League as weak . Every club has a strong enough squad. That is why we often see mediocre teams performing well against big teams and winning.
We have seen West ham perform poorly for a long time. However in the away match against arsenal , West Ham was very tactical and was able to win the match. West Ham also performed well against leicester city and won . Westham is in quite good form now and I think we will see better performance from West ham in the upcoming matches as well.
Teams that are in run to win the title must need to be careful against mediocre teams. Arsenal showed casual attitude towards them, they were lucky to win against Leicester but not lucky enough to win against West Ham United. Arsenal defeat against West Ham united also fade away there chances of winning this season title. Liverpool is going to win this season title because they didn't let there points slip away, they have lost only one game this season.