The sentence you mentioned above can be revised as follows:

Legend has rewarded himself greatly with his several months of assiduous study.

Legend has been rewarded with much progress in English through his a few months of hard study.

Legend's English level has been greatly improved through his several months of assiduous study.

It is noticed that Legend's English skill has been enchanced to such a degree that he can point out a mistake appeared in an English sentence.

reward  vt.奖赏;奖励;酬谢;报答 (英文解释：If you do something and are rewarded with a particular benefit, you receive that benefit as a result of doing that thing.）

reward sb with sth=sb be rewarded with sth

They have rewarded him with $100.

他们酬谢了他$100。

They rewarded the winners with gifts of fruits and flowers.

他们奖给优胜者一些水果和鲜花。

Make the extra effort to impress the buyer and you will be rewarded with a quicker sale. 

多用心去打动顾客，货物就会更快售出。

sb/sth reward sth

How can I reward your kindness?   
我怎样才能报答你的好心呢?

I shall reward his services.   
我要酬谢他的劳绩。

Success rewarded his efforts.   
他的努力获得了成功。

sb/sth be rewarded +(by/with sth)

Anyone returning the necklace will be handsomely rewarded.  

送还项链者将得到丰厚的酬金。

Those who protect the oil field will be rewarded.  

保护油田者有奖。

Our efforts were rewarded by success.  

我们的努力获得了成功。