I think this game is very hard. Tried some fights and lost all, stamina ended really fast.
At beginning we could find easier fights (with weaker enemies)...
It is by design, it gets much easier as you increase your stats.
Stamina increases every time you increase any stat.
Also note the hp of the monster you are fighting if it is over 20 then it is a level 2 or 3 meaning it has higher stats that you can't see.
A level 2 or 3 has a chance to have 20 life too and will destroy a new player, if you lose like 6 fights in a row then there is a good chance that is what happened
I will lower the strength of higher level monsters a little though as I think others may feel the same way but haven't said anything.