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1. Yes 95% of the time, there are some cases where one is better than the other because of attack speed (such as Legione Bosses or bossing in general) but for slaying purposes you're fine with either.
2. I'm sure someone already covered this but a weakness to say, slash basically means a general weakness to melee. If you're using high level weapons (ie Chaotics or Drygores) then you can disregard weakness in slayer because you'll have such a high accuracy it doesn't even matter. Also, if you upgrade your slayer helm it provides an additional accuracy boost so that can help make up for using lower leveled weapons on non-weaknesses. Accuracy is important so you don't miss attacks but if you're using higher leveled weapons then don't worry about it too much <3
Exceptions are monsters like Grifolapines which pretty much have to be killed with Water Spells. It's a rare exception though, if you're ever unsure just ask or check the wiki!
Hope that helps
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2. I'm sure someone already covered this but a weakness to say, slash basically means a general weakness to melee. If you're using high level weapons (ie Chaotics or Drygores) then you can disregard weakness in slayer because you'll have such a high accuracy it doesn't even matter. Also, if you upgrade your slayer helm it provides an additional accuracy boost so that can help make up for using lower leveled weapons on non-weaknesses. Accuracy is important so you don't miss attacks but if you're using higher leveled weapons then don't worry about it too much <3
Exceptions are monsters like Grifolapines which pretty much have to be killed with Water Spells. It's a rare exception though, if you're ever unsure just ask or check the wiki!
Hope that helps