There's not many tips I can give you for this section. It's mainly just how skilled you are at it, so keep practicing if you can't block spears well enough to survive. However, there's still quite a few things you should keep in mind.

-Remember that yellow spears turn around and attack from the opposite direction. Don't get confused by them

-If you get hit, don't freak out; you have a few frames of invincibilitty to recollect yourself, so don't start flailing madly at spears.

-Ignore the red warning on the spears. It usually means that the spear will be the next to come in contact with you, although this is sometimes not correct as it only is based on distance and not on velocity. Also, sometimes the game will purpousley highlight the wrong one to throw you off. Thus, it is better to simply ignore it and get used to using your own judgement instead.

Eventually undyne will release you from your green state and will begin attacking you normally. Each move has its own way to be countered.

-Individual spears: in this attack, individual spears are summoned and thrown at the player. The key to this one is to think of the spears as lines, and to avoid these lines at any time. Remember that the spears move faster than you can, so squeezing behind a spear to avoid another perpendicular attack is a viable strategy.

-Upwards-facing spears: This attack is 3 columns being stabbed by spears. There is no real secret to this one, other than that the spears themselves do no damage after they make contact with the top of the box. Use this to your advantage and strafe between to columns.

-Rotating Spear cricles: This one is tough to deal with. Wait until the spears slow down somewhat, then rush through them. These will continually appear around your soul, so try to avoid being cornered as to leave your options open

-Stabbing spear circles: This attack involves cricles of spears surrounding the player, then all at once being thrown at them. After the spears cross each other, they will not deal any damage, so do not worry about them. The real problem comes whith the sheer amound of them. However, there is a pattern to them in that they alternate positions, i.e. Each spear is located half a spear away from the corresponding spear in the previous circle. Using this, the best strategy is actually to constantly rush towards the edges and strafe around individual spears before they can close in on you.

you can always use flowey's time portal (a website http://crumblingstatue.github.io/FloweysTimeMachine/) to edit the game files.