Hellhounds are a common Slayer task,but are not efficient for XP training. Although they offer decent AFK opportunities and a few valuable drops like the Primordial Crystal,their slow kill times mean you won't make fast progress towards 99 Slayer. Hellhounds also have a very poor drop table—outside of RuneScape gold the occasional clue scroll,Ensouled Head,or rare Smoldering Stone,you won't make much money off these tasks. They can become especially frustrating if you're after the best possible XP per hour.
2. Cave Krakens
Like Hellhounds,Cave Krakens are also high-weight tasks that appear frequently,but they don't provide the best XP rates. These monsters are weak to magic,so you'll need to use a Trident of the Swamp or similar single-target magic weapon to kill them. The process is slow,and their drop table is underwhelming,meaning you won't earn much GP while completing the task. Given their high Slayer task weight,Cave Krakens are perfect candidates for your block list if you're aiming for a more efficient Slayer grind.
3. Black Dragons
While Black Dragons can be profitable due to their dragon bones (good for Prayer XP),they are not ideal for efficient Slayer training. These creatures have high Hitpoints and require you to bring either an Anti-Dragonfire Shield or an Antifire Potion,adding extra prep time to each task. For maximum efficiency,you'll need to use a Dragon Hunter Lance,which is expensive and generally reserved just for Slayer tasks. Since Duradel assigns a relatively low number of Black Dragons to kill per task (around 10-20),it's better to block them and focus on cheap RS gold monsters that can be killed in bulk for greater XP gains.