This study is based on the "2014 the Schweitzer Program" in Taiwan which spanned for four weeks from the 2nd to 29th of August. The lessons included four classes of multimedia picture books and eight game-based lessons. The aim of this research is to describe how to integrate the theory of "Multiple Intelligence (MI)" by Howard Gardner as well as the theories of "Cooperative Learning", and "Game-Based Teaching" into summer intensive English classes among mixed-level and mixed-age students. There were 20 participants composed of 13 fifth graders and seven sixth graders with equal members of each gender. The study followed a qualitative action research methodology with the use of interviews, documents and observations. The students had two 60-minute English lessons and one 60-minute multimedia picture book session per week for four weeks with the researcher. This study provided a prototype of lesson plan designs for a group of mixed-level and mixed-age students. Results show that merging games into the teaching content and using 'Game-based Teaching' is significantly suitable for the mixed group. Based on the results, recommendations were provided for both EFL teachers and EFL schools. During lesson plan design, teachers could merge various games into their lessons especially when intended for students of mixed levels. For the schools, it is best to choose teaching materials that combine text with plenty of activities and classroom games.