>>532682754
There's a lot to unpack here, but off the top my head:

Start playing each class at your own pace in operations. Remember level up your guns, since they persist after you decide to prestige - class reset in exchange for extra perks. Protip for gun leveling: you don't actually need to use a weapon for it to gain XP per operation.

Go for fencing melee just as Brother here >>532683319 explained. It'll help you survive.

You can look up any youtuber for basic builds for each class and weapons' combinations but to get basic gist of how each class rolls they will do.

Learn to mind your surroundings for consumables, you need to be on the lookout for medkits and ammo (especially at the beginning).

Don't do a whole sweep of each area to clear it, that's not necessary.

I mentioned prestige, it's entirely optional after you reach lvl 25, but some perks for certain classes make this system mandatory almost to make the mode, such as siege mode on hard, doable (see fully maxed out Assault who most of the time barely keeps up compared to other classes).