What is the best class in Killing Floor 2 for people who prefer to solo the game instead of playing with others?

I'm on and off playing Killing Floor 2 since its PS+ release last month, and I'm wondering what is the best class to play solo on? I'm currently level 5 in Commando and Level 6 in SWAT, but while I am able to get through a 4 wave match, I always die at the boss.