First of all there are two things going on here. The first is that you have added two extra players to the group. This alone will cause you to have to rethink your tactics. Two players may not seem like that big of a deal but your party’s resources just increased by 50%. By resources I mean actions per round, HP and all the other things players are able to do. This will have a big impact on the game no matter what class the players choose. The game is based on the idea of a party of 4, any variation of that will require adjustment. If instead of gaining two player you had lost two players would you keep throwing the same type of challenges out. What would challenge a party of four will slaughter a party of two.
The second thing you have is the new characters are seemingly more powerful than your existing players. Most of your players are playing classes that do not have to worry about running out of their main resource. Only the wizard really depends on a pool of limited resources. A wizard without spells is basically a commoner with a few tricks. The barbarian does have a limited number of rounds of rage per day, but even without rage they are not useless. The ranger and the rouge are generally effective as long as they have the HP to continue. The magus has two pools of resources he depends on the first being his spells, the second his arcane pool. Once those are gone they become a lot less effective.
You also seem to be unwilling to adjust your play style to account for the changes in the party. With all due respect you have no choice but to adjust your play style. This does not mean you have to completely abandon the idea of the BBEG at the end of the adventure. While you will need to modify that some what you really need to do is to adjust the rest of the adventure. You need more encounters to drain away the resources so that going into the end battle they are low on resources.
You also need to change the nature of the challenges so they have to carefully plan what spell they take. Use challenges that require more than just a shocking grasp to overcome. Most magus spells tend to focus on combat, but they have other spells. Make sure they have some of those spells and create situations where they are needed. Every expeditious retreat or invisibility they memorize is one less shocking grasp they have. The wizard has many if not all these spells so create situations where they need multiple of the same spell. Maybe the who party needs to be invisible to sneak past some challenge.