Hellhounds are among the first monsters that should be considered for blocking. While these creatures are decent for RuneScape gold AFK training and have some valuable drops like the Primordial Crystal,their XP gains are slow. Hellhounds are single-target monsters,meaning that they don't offer the multi-target opportunities that would help you maximize your XP. This can make grinding through a Hellhound task incredibly time-consuming.

On top of that,Hellhounds don't drop much in terms of valuable loot. You may occasionally get an Ensouled Head,clue scrolls,or a rare Smoldering Stone,but these don't offset the amount of time and effort spent fighting them. As a result,Hellhounds are an inefficient choice for a Slayer task and should be blocked early.
Cave Krakens

Cave Krakens are another monster that should be avoided. They have a high Slayer task weight,meaning you're likely to encounter them frequently. But despite the frequent assignments,Cave Krakens are slow to kill. Since you can only kill them with magic (preferably a Trident of the Swamp),these tasks tend to take longer than many other monsters. Additionally,their drop table is relatively poor,so you won't be earning much GP from these tasks.

The high task weight and lack of efficient XP gains make Cave Krakens prime candidates for the Block List. Blocking them will save you a lot of time in the long run,allowing you to focus on more rewarding tasks.
Black Dragons

Black Dragons might seem like a viable Slayer task,but they come with several drawbacks. These dragons have high hitpoints and require special gear to deal with,such as an Anti-Dragonfire shield or an Antifire Potion. To maximize efficiency,you also need to use the Dragon Hunter Lance,a weapon that's extremely expensive and only useful for this task.

While the Dragon Bones are good for Prayer XP,the time spent killing Black Dragons doesn't justify the rewards. Duradel may only assign you a small number of Black Dragons (10-20),which isn't ideal for maximizing XP. What you're really after are tasks with higher kill counts that can be done quickly and with efficient methods.
Greater Demons and Black Demons

Both Greater Demons and Black Demons are single-target,high-weight tasks that can be a nightmare to grind through. Duradel often assigns large quantities of OSRS gold these monsters (130-200),which means you'll spend a lot of time killing them with little XP return. These demons don't drop anything particularly valuable,making the task even more frustrating.