im making this forum cause i've seen a lot of people play certain mercs in an improper way.
Fire support mercs
Arty - on the surface arty may not look like much but he can be a powerful force against the enemy team. First use the artillery strikes as much as you and use from afar. Aim at areas where there tends to be a concentration of enemies (for example in chapel when the EV is moving most of the defending team will tend to group up at the construction site). Just be careful not to take too long as some dude will probably snipe you.
Stoker - I've seen a lot of people blindly throw a molotov trying to get a cheeky kill and let me tell you this wont get you anywhere. Unlike other fire support mercs stoker plays a much more defensive role than others (he can be used on the offensive in certain situations, suppose when the c4 is planted you can throw a molotov onto the c4 to prevent the other team defusing it for sometime). So use the molotov mainly to block of enemy routes. Dont forget to place down your ammo station (btw your can be killed by your own molotov).
Javelin - Javelin is an all round fire support that can be used in any situation, both defending and attacking. Now those who have played javelin will know that her rocket launcher will have two modes: standard and guided. Standard essentially works like a rpg round and will go in a straight line once fired, guided will allow you to control the rocket and guide it to a certain place. Standard is useful if you want a quick kill without being exposed for too long, standard mode is normally useful when there are a cluster on enemies in one place. Guided should normally be used to target someone behind cover. Although in guided mode be carful of being exposed.
Kira - Kira is a very powerful and very fun merc to play as (she's my favourite). When you are using the orbital laser keep moving around as you target the laser to certain locations this will allow you to target enemies without being exposed for too long (btw dont walk into your own laser cause it will kill you, trust me i was an idiot once and walked into my own orbital laser). Dont forget to place down your ammo station for that extra support xp.
i didnt add skyhammer cause theres not much too him and doesnt require a lot of skill to get the hang of him. Hope this guide helped you. Please comment on whether i've done any errors or mistakes in this discussion. If this gets support i'll do more guides for the other merc classes.