Just like other answers, you can't do a task with a lower level range, but you can use boss points acquired to kill the bosses from the lower levels and get the Achievements related to them.
I'm also a charlover and wanted all possible tasks completed, even those that were not good for my vocation, but the only way of doing them now is by killing your character and get to the maximum level required for those tasks.
I've killed myself 10 times to be able to do the Cyclops task from Edron (yes, that's crazy, but it's a charlover thing to have everything completed on the questlog) and to be able to do all possible tasks from Grizzly Adams, I retained my levels until I could complete atleast 1 of each.
As seen in your comments below other answers, the "Killing in the name of.." questline does not mark as "Completed", even if you do all possible tasks in the game. Unless you only want to have all entries in the questlog, don't kill yourself trying to "complete" it because it's not possible.
If I'm not mistaken, all quests that has "repeatable tasks" can't be marked as completed because you can always come back and do the task again. Maybe there are some exceptions, but I guarantee you that most of them work this way.
A few examples:
Krailos Quest, Rottin Wood and the Married Men, Venore Daily Tasks, Goblin Merchant, Spike Tasks, Killing in the name of.., Hive and Liquid Black.. the list goes on..
Here is one question answered on TibiaQA where was asked which quests could be checked as "Completed" if you want to check out:
https://www.tibiaqa.com/19952/there-place-that-shows-all-quests-that-can-marked-completed-the-quest-log