Okay, first, I'm no moderator, but I don't think this is the part of the forum this belongs in.
That being said, here's how I would event this:
Create an event representing the Monster. Give it a second Event Page by clicking "New Event Page." Now, the first Event Command on the first Event Page you must use is Battle Processing, which lets you enter battle with the enemy. The next Event Command you must use is Control Switch (it will be unique for this quest, so name it "MonsterDead" or something relevant to this particular quest.
Go to the Monster's Second Event Page.
Now move your cursor over to Conditions right under the tab and click on the box for "Switch." This will make you select the "MonsterDead" switch.
Now, create an event representing the Mayor. Give the Mayor three Event Pages by clicking "New Event Page" twice. Go to the First Event Page and give him a dialogue asking for help with a monster. This will make it to where he asks for help until the "MonsterDead" switch is turned on.
Now, go to the Second Event Page. Under Conditions, select "MonsterDead" switch as ON. Now give him a dialogue thanking the player. Select Change Gold or Change Items or Change Weapons or Change Armor to give the player whatever reward you want. Then select Control Self Switch, turning Self Switch A on. This will make it to where the player can't go through that dialogue again and collect the reward ad nauseam.
I hope this helps.