Ok, you have a couple options. Leveling up your squadron will help some(disregard this advice if your squad is at max level of course). Youre looking at the lowest level "Routine Missions", but you do still get experience for the "Trainee Missions"(the green ones). And even if you dont really have good odds of winning a mission, your squad still gets half the experience for failing a mission so levels can also be gotten that way. In addition, you can still use your 3 training sessions per day to get more experience as well. And potentially you could drag your squad members into the Command Missions to level them as well, though this requires having a balanced enough team to cover the roles you cant play and to physically play the dungeons with the weird squad AI.

Beyond this, it is possible you would benefit from re-balancing your base squad 400/400. If your team is imbalanced in one way or another then moving the extra points from where you are strong and into your deficient stat could make the difference and allow you to get further down the mission list. While i was leveling my squadron i farmed in chemistries to boost their stats, now that i have my levels and ranks i have eliminated all of those chemistries and replaced them with chemistries that give me rewards instead.

And one final thing. Please dont be offended if this feels like an insulting tip. But have you recruited a full 8 members and are you looking at all of them when comparing stats to mission requirements? If you havent been, when selecting a mission to assign there is a button on the right side of the interface that looks like left and right pointing arrows. This opens your squadron roster, and allows you to choose who to assign rather then just sending the first 4 you recruited. Beyond that, if you really need to change stats then getting new jobs is the move. Every class has a unique mix, so recruiting/switching someone to the right class could help. Need more physical? The tank classes are best at that. More mental? Thats the casters. The best tactical classes are among the physical dps. Finding a balance that works for you helps. Whether thats a combination of multiple classes with varying strengths, or whether it means min/maxing your way into just a collection of the 3 classes that have the highest possible score in one of the 3 stats.