First than all, on Videogames the aggro stand for the parameter on which a mob is based to select their target, on tibia there aren't too many ways to be aware of the lvl of aggro we are generating on a mob till that mob is actually targeting us, worst it seems that some mobs simply have a formula where from time to time they reset their aggro lvls and basically the first enemy which generates aggro on them (and sometimes, the first enemy that is simply on their screen) would be their next target: Giant spider is the perfect example of a mob like that, then we say those mobs "re-target" too often, in the other hand there are mobs that NEVER reset their aggro manually unless their target is unreachable for them (mummys are the best example of those mobs).
Now, there are several keypoints on this thread that i would highlight:
now lets go to talk about aggro triggers:
-Attacking: the most common, each time u attack a mob u trigger it, if that mob have a target on you would chase you, if that mob have a target on other player you only would increase its chances of re-targeting, on this game seems like some mobs simply have a formula that each x seconds they would try to retarget no matter what (GS), and others seems to be more complicated, but all them accomplish something, if you attack those mobs from an unreachable position and they are targeting you, you would trigger a FULL aggro (like rushing through walls), in order to trigger this u only need 2 attacks, now if there is someone else attacking it, the mob would trigger on whoever did the 2nd attack (would need to confirm this last part), also when a mob is restarting aggro, they focus on who trigger more aggro and that mostly means who attack first.
-Facing, most mobs restart aggro when facing an enemy, if who fulfill their triggering is who step in front of them they would focus on it, but there are some moments where they don't do that and keep following their last objective (im almost sure this also have to do with the triggering by attack).
-being at melee: if you are close to a mob when they are restarting aggro you trigger aggro on them and they could simply attack u even if u don't attack them..
then finally lets answer:
triggering aggro on you doesn't depends on the method that you use to avoid being retargeted but on how u deal with the mob once is "retargeting"/restarting aggro
Using bombs: when using bombs the only way to actually trigger aggro on you is with mobs that often to automatically restart aggro or that their main target weren't doing enough aggro on them (not attacking which forces them to restart aggro), when this happens and mobs restart aggro, there is an special mechanism where they choose their target (im willing to say, is the 2nd attack after entering on restarting aggro mode which trigger them)
Joining a bomb to reset aggro: Now, if you join on a bomb after being targeted the mob would enter on restarting aggro mode and would happen the explained above.
using stealth rings: this is an special condition, mobs can't sense you so as they can't sense you, they would never ever target you (of course only few mobs can't sense with stealth ring)
Trap style: if a mob is unable to reach you would restart aggro, if you are in a force field they could enter into full aggro and chase you if you trigger them (giving the 2nd attack after restarting aggro), if you are simply unreacheable at all would retarget into whoever is close to him and even so could trigger full aggro on him.
being out of the screen: here enters the part of sense and seeing you, if the mob doesn't see you but still sense you as a target, would restart aggro and idle EVEN if there is a target close to it, so u would need to step a bit farest till the mob actually retarget otherwise, u could leave the screen, the mob would idle and if you get close too fast it would continue targeting you.
facing creatures: if a creature is chasing someone, and another player face it, would trigger retargeting aggro (so the one facing it would need to trigger the most aggro in order to actually keep the aggro on him)
walking through tank: this is exactly the same than facing the creature, the reason why is almost impossible to get retargeted once doing this is simple: the mob require a path to chase, if the closest path is directly through a player the mob would try it even if that player block the path, so basically would keep retargeting in front of the player until actually chase that player who is blocking its path.
Special creatures: -working on it-
creatures hard to deal without challenging: any creature that outrun when low hp, any creature that often to restart aggro manually (gs for example or medusa and drakens as u said).
How to deal with them: use walls, ALWAYS coordinate your attacks so on every turn both attack equally, also a plan that doesn't work with medusas: step behind the tank.
finally, i hope ppl start researching about the aggro on tibia so at some point we would have more hints about how does it works, my info is almost totally based into personnal experience with few hints from this page retargeting