Formal:
* Христос воскрес! (Khristos voskres!) - This is the traditional greeting, meaning "Christ is risen!"
* Воистину воскрес! (Voistinu voskres!) - This is the traditional response, meaning "Truly He is risen!"
Informal:
* Счастливого Воскресения! (Schastlivogo Voskreseniya!) - This means "Happy Resurrection!" and is a more casual way to say it.
* С праздником! (S prazdnikom!) - This means "Happy holiday!" and is a general way to wish someone a happy holiday.
You can also use a more personal response, such as:
* Спасибо, и тебе! (Spasibo, i tebe!) - "Thank you, and to you too!"
* С праздником! Радуйтесь! (S prazdnikom! Raduytes!) - "Happy holiday! Rejoice!"
The best way to respond will depend on your specific situation.