To achieve at/in something means to succeed or be be successful in/at something.
Examples:
She achieved all her goals in life = She was successful in all her goals in life.
He achieved good results at school = He was successful in his exams at school.
To manage something means to be in charge of (lead a team for example), or to control/handle with skill something.
Examples:
He managed a work team of 10 people = He led and was in charge of a team of 10 people.
She managed all the tasks given to her = She was able to do/handled ok all the work given to her.