CASTLE BOOSTING METHOD
You will need 510,000 EXP for level 50.
If you are a competitionist then I would advise going for this alongside the Archangel achievement as all of those heals will count towards this. However, boosting gives you more Happy Stars, Items and Weapon EXP which help in getting the other achievements.
What you require
Preferably 8 people in a lobby wanting to boost (set up a session on TA!)
At least a couple of people need Items with Team Skill buffs (e.g. Lightning Arrows+)
What will help you progress a lot quicker
Team Skill buffs (Found on Armour) (avoid Happy Miracle or Teleport)
Team Skill EXP buff (found on Helmets)
Conserve AP buff (found on Accessory)
Conserve Team Skill buff (found on Accessory)
EXP booster weekends (keep an eye on TA news!)
Double EXP vouchers*
*These can be awarded for free in game otherwise you need to trade Happy tickets for. You can get Happy tickets free in game or you may have to pay real money for them. The effect can be stacked so if you have membership and use a Double EXP you will get 4x the EXP
Set up a lobby.
Start the match.
Stay in the castle and take turns casting Team Skills to get EXP. The more people partaking, the more EXP you get. A typical score is around 1,600 per game. On a good game you can get over 2,000 EXP in 15 minutes and have even seen some people cap out at 3,000.
Once the game is over, exit out, create a new lobby and repeat.
Hints and Tips (in no particular order)
Not all Team Skills are created equal - Try to avoid Happy Miracle and Teleport as they offer less EXP.
If you launch the Ultimate Weapon Team Skill, hit start and select respawn to get back in as quickly as possible.
Make sure to only launch one skill at a time - be patient, everyone will get a turn and you want everyone in every team skill for max EXP.
For extra EXP, select a Cleric player type and cast Heal after every Cheer and Sacred Song team skill. You can cast Dash or Sacred Armour after other team skills but don't spend too long doing this, make sure you can jump in to the next team skill ASAP. (Heal + Buff can be found on Weapon, Dash and Sacred Armour can be found on Helmet)
For extra EXP a Mage class can cast elemental buffs.
Don't launch the first Team Skill until the clock has started otherwise you waste EXP.
Pay attention to enemies that could interrupt your Team Skill, you may need to launch before they get within striking distance.
Make sure you are actually in the Team Skill as sometimes it can throw you out.
If you are launching the Team Skill use the camera controls (right stick) to make sure everyone is in.
In a perfect world, you'd get 2,000 EXP per game - 8,000 with the premium membership and double EXP vouchers. However, I find that I get more like 6,000 EXP per hour. As a rough guide, this will take around 50 hours without using the premium items mentioned. I got 3 free one day memberships during one of the title updates and have also earned plenty of free Happy Tickets which I've spent on the Double EXP vouchers.
I'm at level 43 which is about halfway with 25 hours of play time.
Be prepared for a grind and bear in mind that getting he Archangel achievement will take considerably longer. I can get 40-50 heals in a good 15 minute game when castle boosting but that's still going to be 50 hours of perfect boosting. Will likely take a lot longer.
Sometimes you will get rushed by the AI and beaten very quickly, other times you will have randoms who will beat the AI but I think around 6,000 EXP per hour is reasonable amount to expect from this method.
Let me know any comments!