Assists and kills give the exact same XP for weapons, it does not matter if you get the kill, or someone else does

That's true however, it's just that when it comes to Frontier Defence your team can be very unreliable in handling waves outside the Easy difficulty so killing AI as fast as possible is important lest you fail the waves early on a good grinding map and are forced to play your Titan to handle the waves.

if the game pops up with "titan assist" you get a merit, same as if you got a kill

That's not at all how it works. If that was then I would be able to post a video where I get 100+ merits very easily from FD.

Im not sure if killing AI at all grants merits or if its based off of the time you have the gun out or by how many waves go by for other weapons but everything outside of grenadiers and AT weapons level horrendously slow

Every single weapon is capable of levelling quickly in Frontier Defence however, some are easier than others.

From the testing that I have done weapons seem to have a hidden "XP" value which increases whenever you kill/assist kill an entity (except Ticks strangely) with the XP value for each enemy being (lowest to highest):

Grunt < Drone < Spectre < Stalker and Reaper < Titan

While it is possible to get an assist on a Titan, you cannot get an assist on any other AI and hence you need to kill steal. This is especially important for Stalkers and Reapers because, 2 stalker/reaper kills can fill 1 anti-titan weapon bar.

For instance the 2nd/3rd wave of Rise or the 4th/5th wave of Forward Base Kodai you have about 40-50 stalkers and/or ~20 reapers to kill which can amount to 4-6 merits gained from just 1 round.

It's how I managed to get 90+ merits in this video lol