A quick guide to Monster Card

This is a quick guide (Not an in-depth guide) to the Monster Card system for new and mid-game players.
 
Number 1: How can I upgrade my skills past level 50?
- A card has 2 different token types. The first one is a Weapon token and the Second one is a Region token. You only need to get the Weapon token to get past level 50. 2/8 card give you the V2 of your first skill, 4/8 for the second one, 6/8 for the third one and 8/8 for the last skill. The V2 of the skill gives a new effect and unlock new runes for that effect and level 100.
Refer to below illustration:
Number 2: Which Region Card is the best?
* For pushing higher stages: Cemetery is the best one with Town Outskirt. Riverside and Frost Hill are both good if you have SS fairies and costumes. 
* For farming: Desert Ruins (much better) and Mushroom Forest (we can't always chose) are excellent here. Try to combine them with Cemetery and Town Outskirts if possible.
Number 3: How to get keys?
* From stage 0 to 600, you get No Keys,
* From stage 700 to 2900, you get Bronze Keys.
* From 3000 to 5900, you get Silver Keys.
* From 6000 and above, you get Gold Keys.
You can only get the keys from every 100 stages 1 time per return. Meaning that if you get the gold keys from stage 6000, fail and beat back stage 6000, you won't get it again this return but will have to wait the next one.
==How do I build my deck?==
The most important thing is to have 8 cards for the weapon you are playing.
Once that is done, you want to try getting good region bonuses. 4/4 Mushroom and 2/4 Cemetery is the core meta at the moment if possible. Since they are good without full SS and has the best multiplier. But not worth obsessing over if you don't have good leader as mention below.
For new player, Frost Hill, Riverside and Town is not a big buff since you likely not having Full SS fairy, SS costume and SS Gear.
After that, look out for cards with a notable leader effect (explanation below)
And lastly, take into account the basic stats of your card, you want offensive stats as much as possible. 
This doesn't mean you should sacrifice 4 cards with great stats just for a slightly better region bonus, but all things being equal, this is the order of what to look out for by order of importance.
Side note: if you want the best Combat Power Deck, go from Bottom to Top.
==Leader Effects==
The 4 monster cards you put in the top row of your deck will have their "Leader Effect" activated.
 
There are few notable leader effects that are really good (in no particular order):
 - Trained ATK x xtimes / Excellent leader effect for pushing higher stages, can make a huge difference in power.
 - x% additional damage on boss monsters / Strong bonus both for pushing and farming.
 - Empowers an ability of xregion / This make the card give an additional token of said xregion in addition to its own base tokens (i.e. weapon, region). Very convenient to have more region bonus than you could normally have.
 - Critical/Skill critical chance / Hard to get critical chances much past 50% with other sources, decent leader effect.
 - Reduce CDR for "X" weapon / This is pretty self-explanatory. This leader buff can make some weapon has 0 CD cough cough Bow cough cough . So you better get as much as you can.
Guide Thx to Nate & Edenlight
==Monster Cards Library==
If you want to have a better idea on your future card deck builds, there is an existing tool named Monster Cards Library which was made available for this purpose. 
Monster Cards - Library (link)
Click the image or URL to be redirected to TDS Monster Cards - Deck Simulator created by @Natelicious. 
* Note: Inter-wiki and external links are being loaded on the same page, better open link in a new tab.
==Monster Card Deck Builder==
This is a new Unity WebGL Player-based tool named Monster Card Deck Builder which utilizes the Monster Cards Library as its data source. 
TDS Monster Cards Deck Builder (link)
Click the image or URL to be redirected to TDS Monster Cards Deck Builder created by @Spark. 
* Note: Inter-wiki and external links are being loaded on the same page, better open link in a new tab.