I replaced my magamatas that giving %18 sword affinity and 5 slice element with Snail magamata that gives 18 slice element and %5 sword affinity.
Simply my normal attacks just increased 8-10, (i hitting yangyu-san's 225->235 with first hit) so whats with slice element and is it affects skills?

You do more damage against enemies weak to slice element
Have to check your bestiary for that

so if you're like going in a dungeon where you know their weakness you would want to switch magatamas

I'm guessing swords are all slice and bows/spear are pierce? What about the thrust skill with the katana? I forget the name.

I checked, the mobs i killing have not weakness to slice element. Slice element have something with some weapons?

Slice elements are the weapons: Axe, Sword, Dual sword and Odachi.
Pierce Elements are the weapons: Bow, Spear, Wand and Staves(Staff).

^this two actually blend with the elemental affinities so...

[feel free to correct me though, I only got the idea by testing them in a small amount of time.]

From the Wiki Page:

Styles
Styles determine the type of force a weapon inflicts upon an enemy: slicing, impact or piercing. Different youkai has different resistances to each.

Slice: Slice, as its name indicates, is an attack where a blade cuts the enemy. The katana, odachi and twin blades are the weapons that often offer this kind of attack style.

Impact: Impact, as its name indicates, is an attack that uses brute force to inflict blunt damage. The axe, wand and staff are the weapons that often offer this kind of attack style.

Pierce: Pierce, as its name indicates is an attack that attempts to stab the enemy with a sharp attack. The bow and spear are the weapons that often offer this kind of attack style.

One other attack style is the "multihit" style. But this is a style more related to abilities, not the basic attacks.

it gives a multiplier to ur auto atts and skill atts. a 18slice element means 18% damage increase on ur auto and skill atk for ur swords. while affinity is a flat increase in atk.

Slice Element increases your damage for slice based skills (Some auto attacks and any skills with full or partial slice element as aprt of its damage) by the % indicated.