Guide on upgrading and stat bet
1. You cannot use stat bet or upgrade skills on normal monsters, with the exception of legendary monsters. Non-Legendary Monsters have to be bronze grade and above to do these two things.
2. So how does this upgrading system work?
Firstly, you need a level 50 S grade monster as your main monster to be upgraded.
Next, you must have another same monster (must be same evolution too) of any grade or level to sacrifice into your main monster to make it bronze.
To make a silver monster, you have to combine a bronze monster with a same monster which is level 50 S grade, and to make a gold monster, you have to combine a silver with a bronze one.
All of these monsters have to be the same. Different evolutions do not work too.
Upgrading costs:
Common- 20000
Uncommon- 50000
Rare- 100000
Epic- 1000000
Legendary- 3000000
So altogether you need 5 of the same monster (two of them level 50 S grade) and 4million silver for a gold epic monster for example.
3. When your monster has reached bronze grade and above, it can do stat bet. Higher grade monsters have a higher stat bet cap. Here are the stat bet caps below:
For Common-
Bronze: +15
Silver: +30
Gold: +45
For Uncommon-
Bronze: +15
Silver: +30
Gold: +45
For Rare-
Bronze: +15
Silver: +30
Gold: +45
For Epic-
Bronze: +20
Silver: +50
Gold: +80
For Legendary-
Normal: +20 (means unupgraded legend can bet until max of +20)
Bronze: +40
Silver: +70
Gold: +100
4. So how does this stat bet work?
Basically, you choose from 3 options to make a bet from.
The bets give a chance of raising or lowering your stats up to a maximum and minimum (original stat at level 50 S grade). You can choose to either keep or abandon your bet so bad bets won't make a difference to your stats. You can bet as many time as you want in a day.
The normal bet costs 10k silver, Super bet costs 2 essence of life and Ultimate bet costs 10 gold coins. Obviously the more expensive the bet, the higher the chance of getting good bets.
If you reach the bet cap, your stats cannot rise any further until you upgrade your monster to a better grade. E.g from bronze to silver.
5. Skill upgrade
When your monster reaches bronze or higher, you can upgrade some of their skills to have higher power, or in the case or supplementary moves, greater increase in stats.
Upgrades usually increase the power of skills by 5 points apart from unusual skills and aoe skills (increases by 2/3 power each skill upgrade). The max you can upgrade your skills is 6 times, so a skill like thunderstorm+6 will have the same power as super thunderstorm when fully upgraded! And without stunning itself too.

6. Cost of upgrading damage skills
+1: Skill Book of the skill you wish to upgrade, 100 Essence of Wisdom
+2: Skill Book of the skill you wish to upgrade 100 Essence of Wisdom
+3: Skill Book of the skill you wish to upgrade, 150 Essence of Wisdom
+4: Skill Book of the skill you wish to upgrade, 150 Essence of Wisdom
+5: Skill Book of the skill you wish to upgrade, 200 Essence of Wisdom
+6: Skill Book of the skill you wish to upgrade, 200 Essence of Wisdom
Supplementary Skills
+1: Skill Book of the skill you wish to upgrade, 300 Essence of Wisdom
After upgrading to Bronze and above, Epic monsters and above can continue enhancing from S to SS grade.
That's about all I know for now. Hope this has cleared some doubts.