General tips

Attack types:
There are 3 general attack types you can find:

Normal attacks:
Normal attacks deal damage when the player is touching it. There is one subclass: piercing. Piercing attacks will go through the enemies. All piercing attacks, except bone zones, inflict karma effect (?).

Blue attacks:
These types of attacks always pierce (?), unlike normal attacks, players can avoid taking damage from it, if they stand still. Jumping, spinning and dancing is will not result in player taking damage.

Orange attacks:
These, work same as blue attacks, except they don't. They always pierce, but the player must move when touching these types of attacks in order to not take damage or not touch them at all.

Combining attacks:
When playing as a monster, you can combine attacks for stronger attacks in order to inflict guaranteed damage.

Bone clip:
When playing as lost bread last breath sans phase 1, you can time your bone walls in a way, so they will clip through each other, this will result in guaranteed damage. This is hard to do, but with enough practice and skill, you can make this every time, and be a lot stronger in phase 1.

Cross-platform:
When playing as Dustnuts Dusttrust Sans phase 1, it is better to save up your cross blasters until your bone flood recharges. Cross blasters are hard to dodge in such limited space. And remember, this ain't your standard bones, it is blasters, they will deal heavy damage.

Laser hell:
When playing as Dusttrust Sans phase 2, using your cross blasters, multiple blasters, all slashes and normal gaster blasters, will result in a ton of lag lasers. And actually, lagging the game, may result in it being harder to dodge stuff.

True bone wall:
When playing as Swap! Papyrus, it is a great idea to use his bone walls (the ones that go from one side of corridor to another) at the same time, might make them be on each other, and make unavoidable damage.

Jumper's problem:
When playing as Swap! Papyrus, using bone walls when enemies are under orange soul effect, it will be very hard to dodge the bone walls 'cause of constant jumping.

Spamming:
When playing as literally anyone (except humans), mashing all of your buttons and abusing the powers of your mouse will make many attacks happen at each other, making them hard to dodge. It is also possible to spam the same attack that has a small cooldown until some of your stronger moves recharge, and still deal some easy chip, damage.

Spamming VS Combining
This question's answer is based. Based on the character and the gameplay you want to do in general. The bigger moveset and cooldown the character has, the better it is to spam.

Pacifist! Sans
Just play Swap! Sans

Swap! Sans
It is generally easier and more effective to spam bone swings and blue bone walls.

Papyrus
What did I say about Pacifist! Sans? The same applies here.

Sans (phase 1)
You can't even do anything good. Even with spam, even with combining. Why? There is nothing to spam and combine.

Sans (phase 1.5)
Spam is the way to go most of the time.

Vones sans (phase 1)
Same thing as sans phase 1 applies here, except, a little better.

Vones sans (phase 2)
Spam.

Swap! Papyrus
This character is walking spam. His attacks are spam. His gameplay is spam. Also has an attack that literally makes spamming more effective. Truly the king of spam.

Horror! Sans
Well, i mean, there isn't much to spam.

Last breath (phase 1)
Combine. Combine 'cause bone clip go brrrrr.

Last breath (phase 2)
Spam. 'Cause many moves and they recharge quickly.

Last breath (phase 3)
Spam. For the same reason as phase 2, but also you must kill people as soon as possible, or you are dead.

Racer sans
Spam. Spam all the movement buttons.

Bad time duo
In general, it is better to combine, but this is unlikely to work, unless you got a good teammate, or you are speaking to him on discord.

Ink! Sans
You are most likely to get attacks that don't go well with combining. So spam.

Among tale
sus. Amogus. (the gameplay is just none of these)

Dusttrust (phase 1)
Combining is usually better.

Dusttrust (phase 2)
Spam. Despite the fact he has a good combined attack, ⁣just don't use them when they recharge.