Idioms:

Jekyll and Hyde noun [C]
someone whose personality has two different parts, one very nice and the other very unpleasant
You can't depend on him to be friendly - he's a Jekyll and Hyde.
Many alcoholics develop Jekyll and Hyde personalities.
The professor was a real Jekyll and Hyde - sometimes kind and charming, and at other times rude and obnoxious.

like oil and water
at first my boss and i were like oil and water.

blacken sb's name/character/reputation etc. to say things about someone that are not nice, in order to make other people have a bad opinion of them

be no big deal INFORMAL
to not be a serious problem:
It just means we'll have to pay a little more - it's no big deal.
I'd like to work out today, but if I can't it's no big deal (= not important).
a) said in order to show that you are not upset or angry about something that has happened:
• It's no big deal. Everybody forgets things sometimes.
b) said in order to show that something is not important:
• The kids won't get as many presents this year, but that's no big deal.

big deal
something that you say to show that you do not think that something is either important or interesting
'Did I tell you Ann got a new car?' 'Big deal!'
feel/look like a million bucks/dollars INFORMAL to look very attractive or feel very happy and healthy:
• I felt like a million bucks in that tux.
I felt like a million dollars in the tuxedo at the President's reception yesterday .
Look! She looks like a million dollars in that gorgeous dress today.

Helen came back from her winter vacation feeling like a million dollars

give sb a (big) hand to
1-CLAP1(1) loudly in order to show your approval of a performer or speaker
Please give her a hand This speaker always gets a big hand
2-Give someone a hand :Help someone when he/she is doing something difficult. Also, lend a hand. Help a person,
Let me give you a hand with those chairs, or Jane is always willing to lend a hand with refreshments.
Let me give you a hand with your baggage.
Don't worry about your garden. Ten young people will come over today and give you a hand.

money doesn't grow on trees means that it is not easy to earn money, so it is valuable, and you have only a limited amount of money. You should be careful how much money you spend.

A: "Mom, can I buy a new dress?"
B: "We can't afford one now. Money doesn't grow on trees."
Our lab computers are getting old, but we can't afford to buy new ones this semester. Money doesn't grow on trees, you know.

feel like jelly also turn to jelly if your legs, knees etc. feel like jelly, they start to shake because you are frightened or nervous:
• When the boss called me into her office my legs felt like jelly.