This guide is for the 1.0 version of the game

This is the strategy I used to unlock all characters (last tier needs to MVP on hell difficulty with each character)

Beating the First Boss:
Using these builds I usually need 1 Fighter (for tanking) and 1 DPS. (Note: Review below Section for Berserker and Boxer exceptions)
I will usually add 1 healer base class also then save the class items to combine for the advanced classes (You can combine 1 of each class you don't need to guaranteed get the remaining class if you are having issues at the end of the run getting enough to combine)

I put all Offensive upgrades on Main DPS, put the Health upgrades on the Main Tank at the start but get the Main DPS / Offtank up to 3000 health when you start getting attacked on both sides
For Defensive upgrades I get the Tank above 100 then start giving some to the Off Tank / DPS.

I don't use Anger (Advanced Class) much as it is melee and I find it worse than Firefist and Sorcerer
I didn't unlock the skills Sacrificial Fortitude or Bio Skin as I didn't think they fit into my builds and I wanted to keep the card pool smaller, but I usually easily get the whole build by the end of the game so this is not required
When upgrading the shop it rerolls the skills so be sure to buy all the skills you want before upgrading. When you do upgrade the shop you get a bunch of free rerolls so use these before spending gold on rerolling.
Always fight purple levels (Except if you are low on souls in the last zone)

When referring to a defensive skill in the future I mean one the following:
Taunt
Raging Impact
Frozen Aura
DMG Amplification
Defense Enhancement
Speed Boost
Furious Punch
Furious Power
Turtleborn
Note: By endgame I want at least 1 copy of all of the above at level 3 for combinations or use

For Character Selections pick Fighter + Priest + Idol + Any DPS Class (Note: Review the below section for Berserker and Boxer exceptions)

Tank - Fighter class - 3 Defensive Skills
Healer - Beginner or Idol class - Defensive or Healing skills (Basic Healing, Healing, Gentle Rainfall and Heavenly Song)
Main DPS - Class of DPS - Skills listed in sections below
Off DPS - Rockie class - Range DPS's skills or skills you will use in the future that wont fit on other characters

Tank - Veteran or Templar class - Furious Punch, Power of Black Tortoise EX, Frost Taunt EX
Healer - Apostle or Star class - Stronghold Healing EX, Swift Dew EX, Celestial Melody EX
Main DPS - Advanced class of DPS - Furious Power (to avoid 1 shots and help getting 3000 health), Other skills listed in sections below
Off Tank / 2nd Healer - Veteran/Templar/Apostle or Star - Secondary Copies of Heavenly Song or Defensive skills (whatever shows up a lot for you in the run)

Note: If you do not have the EX skill unlocked, just pick one of the ingredient skills for that slot instead
Note: You can replace an offensive skill with Overload on any character

Assassin/Ninja:
Notes: Scales off Critical Hits, using 3 offensive skills so be sure to give him some HP and Defense upgrades
Offensive Skill 1: Shadow Dagger + Frozenblood Veins > Frost Dagger EX
Offensive Skill 2: Inertial Strike + Frozen Strike > Frost Blast EX
Offensive Skill 3: Overload

Archer/Marksman:
Notes: Scales off attack speed
Offensive Skill 1: Gentle Breeze + NorMore Attacks > Storm Rage EX
Offensive Skill 2: Ice Elemental Thrust + Icy Multishot > Ice Scatter EX

Mage/Sorcerer
Notes: Scales off casts and exclusive skill casts very fast, the strongest class in my opinion
Defense Skill: Frozen Burst + Frozen Amplification > Frost Burst EX
Offensive Skill 1: Magic Bullet Scatter + Frost Shackles > Frost Scatter EX
Offensive Skill 2: Burning Fireball + Flame Burst > Flame Burst EX

Pirate/Captain
Notes: Scales off physical casts (typo in game says magical, but its actually physical) + fire damage. Doesn't scale with attack speed
Offensive Skill 1: Bomb Throw + Flameblood Veins > Fire Blood Bomb EX
Offensive Skill 2: Melting Shot

Knight/Lord
Notes: Scales off casting (magical or physical), normal attacks and ice damage
Offensive Skill 1: Frost Charge + Frozenblood Veins > Frost Thrust EX
Offensive Skill 2: Ice Elemental Thrust + Icy Multishot > Ice Scatter EX

Lightninger/Thunderer
Notes: Scales off Thunder damage, Spark and a bit of Fire Damage.
Offensive Skill 1: Lighting Chain + Explosive Shot > Lightning Fire EX
Offensive Skill 2: Flame Burst + Thunder Orb > Thunder Blast EX

I have found these two classes don't work as well in my regular setup as a Fighter + Berserker/Boxer + Rockie + Beginner composition has a really hard time beating the first boss.
Instead I like using both together as hybrid Tank/DPS setups (with Berserker main tank) with two healers (Star + Apostle)
Recommended to go Fighter + Berserker + Boxer (if you cant get both Berserker and Boxer before the first boss use an Idol) early then hold out for the advanced classes.
Apostle or Star should be setup the same way as the normal build, and just get as much duplicate healing as possible on the other one (only use Furious Power defensive, rest are melee range)
Berserker gets most upgrades, try and keep both Berserker and Boxer's HP up (with Boxer slightly higher as he has less defense)
Notes: This build is good at getting the no deaths achievement

Since you don't really need Taunt and you dont want to lower your DPS with turtleborn, there is less defensives you want to take, still equip one per character
Defensive Skills:
Raging Impact
DMG Amplification
Defense Enhancement
Speed Boost
Furious Punch
Furious Power

Berserker/Champion
Notes: Scales off taking damage and physical damage
Offensive Skill 1: Flame Whirlwind Slash + Thunder Orb > Lightning Slash EX
Offensive Skill 2: Inertial Strike + Frozen Strike > Frost Blast EX

Boxer/Firefist
Notes: Scales off fire damage
Offensive Skill 1: Blaster + Dragonkin Bloodline > Dragon Flame EX
Offensive Skill 2: Fireball + Flame Burst > Flame Burst EX

Alternate: Pick DPS + Berserker (Tank), Priest and Idol as use Templar (Fighter + Priest) as off tank (Don't have to pick Fighter in this case) (Use Tank build on Templar but use extra heal instead of Frost Taunt. Still use DPS build on Berserker)

You must MVP elite/hell with a class to get access to the advanced class
You must MVP Hell with an advanced class the get the EX of their exclusive skill
Other EX skills are learned from the Talent menu
Lightning EX skills are learned by killing the different Dark Lords in Hell