Assassins are easy to kill once you gain enough experience with the game (not level experience, real experience). Even with gold armor they're very fragile and most weapons can rip them apart in a split second. Spotting them also isn't as hard as some of the nub assassins believe, when they're cloaked they're LOUD and when they're somewhat close to you their cloaking gets weaker so you can easily spot and kill them if you hear them approach. The tank is by far not the only class who can kill them easily. The sniper has a deadly grapple and a high DPS SMG, the support has his shotgun that assassins love to run into, the assault can use his rifle or just charge-grapple the assassin out of the ring and the gunner's minigun just murders ANYTHING that gets close (for laughs you can also ground slam to toss the assassin off the map).
There's no inherent advantage to being higher level in MNC, the only unlock that makes you better at the game is your first custom class slot (costs 15k, average payout per match is 3k) because it lets you stick your preferred endorsements on your pro, after that more custom class slots are only for convenience. All endorsements are unlocked from the start though the files look like they were originally supposed to cost money to unlock. Joining a game in progress is a bit bad because you didn't get to build up the money to upgrade your skills but for the most part everything is down to player and team ability.