For bosses, you pop your cooldown either right before you know a tankbuster&mechanic is gonna hit you. Usually you pop your best cooldown but by learning the fights and doing them more often you may realise that a smaller one may be also enough, especially if you know that something harder comes along later.
For pulls, you gather your mobs and once you decide to stop you pop your cooldowns. Usually you will also start with your biggest one but it also depends on how many mobs you pull. If this is a small pull mit maybe 2-3 mobs and a larger pull will be coming right after you may save your 30% cooldown for that big pull later. If you pull big from the beginning start with 30% and go down that road once they run out of duration. In the beginning you can (and should) also throw in smaller CDs like arms length or Reprisal together with your bigger CDs.
If you are playing PLD and have holy ground you can and should also use it on big pulls as your first big CD, when you take the most DMG. Don't use it just as an emergency button.
The thing is more about your equipment and the equipment of the party. You can do this in most dungeons but you may not survive this if you're not equipped for the task. Same is true for the healer as they won't be able to keep up with the incoming dmg if their equipment is lacking. And also if your DDs won't do good damage the mobs will take forever to die and you may run out of CDs. It's a group thing so make sure your group is okay with the big pulls, especially if there are newbies in your teams.Wall to wall pulling, is this a thing I should do in level 50 dungeons or wait until higher levels.
Yeah, most of the time you want to hit everything because you don't want to lose aggro.Should I spam aoe until there’s only 2 enemies left then go onto single target attacks.
That may be a good idea to test the waters, especially if you are not sure if your team is equipped well enough (see above)Do you usually pull one pack and see how you get on and that determines how you will play for the rest of the dungeon?