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When do melee fighting, I usually just spam Increased Damage because it causes a lot of damage, of course. But I have 3 other skills that I try to mix in there, but I don't notice any bonus in these.

Bleed is suppose to do Damage Over Time if I'm correct, but I usually don't see it.

Ignore armor I'm sure is useful against enemies with tough armor, but I have rarely had a moment in which it's useful yet.

And accurate attack sounds good, but I don't miss that often.

So, in most mob slayings with melee, I just spam Increased Damage.

Am I doing it wrong? What situations will arise where these skills will be useful?

There really is no wrong way to use skills. It's your own way that you find best.

Usually I don't think bleed is very useful until higher levels. Are you watching system info when using it because you will see a message that says the entity bleeds a certain amount every few seconds.

Ignore armor is only used for pierce weapons. In system info you will see the damage you are doing. I'll use an example. You might see 232(232) damage that means you are doing max damage possible, but if you see 152(232) that means the critter is resisting some of your attack. This is the case where you would want to use ignore armor so that the naminal that use to be hit for 152(232) will now be hit for full damage at 232(232).

Accurate attack helps immensly and I usually use it in every single one of my melee actions.
Hope this helped a bit.

I'm under the impression that Accurate Attack is more helpful against mobs that dodge and are about your level or higher (so they manage to dodge against you) than against mobs that parry or are of a lower level. Is that right?

Accurate attack is for hitting things of a higher lvl. Hunting lower you can use more increase dmg or credit lower to conserve stats. What I see most from all players is the waste of stats i.e. giant bomb heals cast on a single target or a 3K nuke on a lvl 50 gingo or a tank beating themselves to death.

Right, accurate attack increases your attack level. It will be useful if you hit (a) targets of higher level and/or (b) targets that don't match well with the adversary dodge/parry of your weapon. Using 2-handed weapons it is generally useful when hitting targets that dodge (because adv. dodge isn't good on any 2h weapon). Using Sword+Dagger it is generally more useful when hitting targets that parry (because these weapons got excellent adv. dodge and average to bad adv. parry).

Bleeding can be used with slashing weapons (sword, axe) and triggers every time you do maximum damage.

Eh, it's just that double damage is always useful while the other stanzas are for specific situations.

Bleeding only works on full hits, and since it's gradual it's hard to see the effect if you're not looking at sysinfo. The extra damage is about comparable to increase damage, the real use of bleed is that it can be stacked with increase damage.

Ignore armor is only going to be usefull against mobs with armor against piercing, which is only about 40% of the mobs. You'll be thankful for it if you run into such a mob though.

Accurate attack has been explained already.