I will talk about Maggie and Griffin, as I have both of 'em on Elite right now (Abe is the boring one, so he takes the 3rd place for Elite podium).
As a general rule, If you have a healer (not Lazarus) in your team I would suggest taking swap speed to increase the DPS of your team. Basically, you switch to your tool to place it or use it, then switch to the gun to deal some damage in the meanwhile tool reloads.
Place the traps away from where people are running, that’s because monster is less likely going to break them before they are armed. If you place them where your teammates are running around, monster has a chance of using AoE attack that will destroy them before they will doe any good. Put them on the side of your path, on some small cliffs, space them out.
Placing them around a corner, where they have limited view range isn’t a bad idea. Sometimes you don’t want traps to fire from that location if monster isn’t in a specific way. You prevent that by blocking its view with obstacles.
Now, reload speed isn’t that bad if you want to stay 24/7 on the harpoon, however you are sacrificing a lot of DPS by doing so. Harpoon in itself deals some damage, however Griffin’s SMG is the most accurate weapon of all trappers, it’s basically a laser gun. Swap speed is recommended. Use harpoon when you see monster is focusing on someone.
Never fly in the air if you are going to harpoon a monster, as harpoon won’t do its job, you must be on the ground for it to stop monster in its tracks.
Firing harpoon to the face of a monster is not a good idea, he can still close onto you, he just can’t move away from you, this doesn’t help if you are the one being focused. If you are in that state, ask Val how to deal with monster aggro where you have no tools to escape (merry-go-round).