The direct translation for 'much better' into Spanish is 'mucho mejor'. You could also say 'estoy en buena salud' which means 'I am in good health'. 'Salud' is the Spanish word for health and is often used as an informal toast.
There are a number of Spanish phrases you can use toask about someone's health, or to indicate good health or otherwise:
How are you? - ¿Cómo está/s?, or ¿Qué tal?
How are you getting on? - ¿Qué te cuentas?
I am like a cannon! - Estar como un cañón (an expression of good health)
I'm as healthy as an apple - Sano/a como una manzana (as above)
I am over-ripe - Estar pachucho (an expression of poor health)
I've turned into dust - Estar hecho polvo (as above)
I have a headache - Me duele el tarro/dolor de cabeza
I have a cold - tengo un resfrio
Of course, it's easier to learn Spanish words and phrases when you know how to pronounce them, and this film lets you do just that. Salud!