Here's how to receive the 'happy' ending complete with unique credit sequence:

1) Answer 'It isn't the case, Kensuke.' (2nd choice)
2) Answer 1st choice
3) Answer 'Look at the transfer student.' (1st choice)
4) Answer 'Look at the transfer student' (2nd choice)
5) Answer 'Goto library' (1st choice)
6) Answer 'Help transfer student pick up her books.' (2nd choice)
7) Answer 1st choice
8) Answer 'Look at Misato.' (3rd choice)
9) Answer 'Short range attack.' (2nd choice)
10) Lose the battle then you will pick up Megumi and move her away from the battlefield. After moving her, win the battle.
11) Answer 'I understand.' (1st choice)
12) Answer 'I cannot forget today's battle.' (1st choice)
13) Answer 3rd choice
14) Answer 'Go to NERV HQ.' (1st choice)
15) Answer 'You really like books a lot, eh?' (1st choice)
16) Answer 'I read alot of books.' (2nd choice)
17) If Asuka asks you to answer for her, select choice 'C.' If not, skip to the next step.
18) Answer 'Invite Megumi to sing.' (1st choice)
19) Answer 2nd choice
20) Answer 1st choice
21) Answer 'Long range attack' (1st choice)
22) Answer 'Do not kill Megumi.' (1st choice)