General tips:
Buy a Sea Tea from Gerson (the turtle shopkeeper in Waterfall)! It's really helpful for the 2nd section. Use it when he spares you, since it doesn't heal much, and you don't want to waste a turn drinking it.
Here's the foods you should try to have. (If you don't have them, don't worry. You can beat him with a legendary heros.)
Butterscotch-cinnamon pie
Instant noodles
steak in the shape of mettaton's steak
snowman piece(s)
Legendary heros

Try not to use ANY food before he spares you, since you can eat as much as you want then.
Also try to have the burnt pan, as it makes your food heal 4 more HP. It's in hotland, and you can backtrack to get it if you didn't throw it away.
Attacks:
First Half
"The strongest attack"
Just memorize how it works. You should try not to get hit at all during this one most of the time.

"Jump Rope"
tap the up key very lightly, and press it right when you land.

attack with lots of short bones going across the floor, and lots of long bones going across the ceiling in the opposite direction
Don't jump over each individual short bone! Follow the top ones across (left?), then follow the short ones the other way and repeat.

"Blue Bones"
Make small jumps over the white ones, also moving slightly with them, repeat.

Mixed height jump rope
Jump right when you land, and hold up for a time appropriate for the height of the bones? xD

Ones with platforms where you have to go either left and right (the ones that are always the same, and are platforming type ones)
Memorize them! xD

3 levels with bones moving with platforms
Stay on the bottom level as much as you can, and just move to the next level to dodge ones on the bottom level.

One with 3 columns of bones, where you're on a platform that moves right then left
don't jump very high Hitbox dodging is very helpful xD

Gaster Blaster 3 level one
Again, mostly stay on the bottom level unless there's a blaster on the bottom level. Then go to the 2nd level, and return to the bottom one once it's clear.

Second Half

Menu Attacks
Try to remember what option you used last turn. Press left right after the attack ends if you were on fight last turn.

Clips (one where it jumps between mini attacks)
Most of them involve jumping, so be ready to press up. Look out for Gaster Blasters, as it can be hard to dodge those ones. This one's just reaction speed..

Aiming Gaster Blasters
Go in a sorta circle shape, and move towards the blasters, not away from them.

Blue Slam
Watch his hand, not your SOUL! He points before your SOUL's direction changes. The really fast one is probably the 2nd hardest attack to dodge.

Opposite vertically moving bones
move up or down right away, then go in very small circles. I had a lot of trouble with this one.

"Alright survive THIS and I'll show you my special attack!"
The hardest attack to dodge (ironic how he doesn't understand why people don't use their strongest attack first, but then uses his strongest attack last, like stereotypical bosses xD)
Try to memorize this one too. Try to go into it with nearly full health. Sea Tea makes the circling Gaster Blasters SO much easier. It's really frustrating when you die on this, since you're SO close, but it is the hardest!
Once he starts trying to slam you on the walls (without any bones), you've won! (Slamming you against the walls doesn't do damage if you're at 1 HP)

If you're really having a hard time, you could watch the fight online and see their strategy. You can watch the fight without seeing the ending.