As mentioned above, characters sitting in the wagon will receive experience points even if they did not participate in battle. (dragon-quest.org)
I highly recommend that you take your time to participate in these battles between main events as not doing so will leave you incredibly weak as the difficulty quickly increases. (gamerassaultweekly.com)
Ultimately, higher statistics mean an easier time fighting, since you really do not actively participate in battles. (thatvideogameblog.com)