Short version :
Your giants are essentially (oversized) tanks. Use them like tanks. Kill them like tanks.
giants are ~16m tall
Let's start with size and armor.
At 16m tall and roughly human shaped you'd expect their width to be something like 2m. Battleship armor was typically around 2m thick. So the would be completely unable to move.
Human armor (for a knight) would typically weight of the order of 45 lbs ( so about 20 kg ) and between 1mm and 3mm thick (varying over the body), so let's use 2mm on average.
The giant should be able to manage thicker armor, but only really in proportion to it's height, so maybe 8mm-24mm thick.
For reference a an early WW2 Panzer III tank would have had a max of 30mm armor, and many tanks had less at the start of WW2. So your giant is in pretty good shape with e.g. 10mm plate armor.
giant weapons are essentially large caliber naval guns modified to be fired by a giant (single shot)
A human might carry e.g. a gun weighing, say, 4 kg and that's for a man weighing something like 80 kg (and these are really rough figures). So gun weighs about 1/20 th of the human's weight.
So scaling that up, your giant might be 16mm tall, but the mass will be something like 512 times that of a human. So the gun can be about 2000 kg.
That sounds like a lot, but two problems arise.
Firstly recoil. A big gun will have a huge recoil. Ever see what happens when a tank or battleship fires it's main armament ?
Which is why human guns fire small bullets ! A 0.45 caliber bullet would be roughly 16 grammes. So your giant, even with a 512 multiplier, is only going to fire slugs that are at most 8 kg each.
A more realistic type of weapon (if you insist on a large caliber artillery type piece) would be e.g. the British 15 pounder which had a barrel and breach weight less than 2,000 kg and fired 6.4 kg shells. This was a WW1 weapon.
You probably want to use some kind of armor piercing round and equip your giant with high explosive rounds (a mix, like a modern tank would).
a world where a small number of giants coexist rather well with humans
So we have a large armored "vehicle" with a modified artillery weapon.
There are a relatively small number of these "vehicles" so the majority of your troops are human, with the giants (i.e. the "tanks") deployed in whatever manner your tank strategy dictates.
Giants can also perform recon (16m tall and a telescope and not much gets hidden easily :-) ).
Humans are ideal for e.g. entering buildings, ambushes, killing other humans (they ought to be good at it by now) and doing those logistics things your giants could be spared from.
Which is where the the problem lies.
At 16mm tall your giant is more or less oversized for the firepower you get back. They'll need a lot of support resources (like food) and "repairs" (medical care) will be a challenge on a battlefield.
But assuming you can work around that, you have large tanks that will also perform useful functions like recon.
Some giants will be dedicated to anti-giant roles (guns designed for killing armored giants, probably less useful for human support roles). Some will be scouts ("run over there have a look") and heavily armored but not armed much. Some will be APCs, some will be a kind of main battle tank.
In their APC role they'll be carrying humans (and 10 humans per giant is a rough estimate).
In other roles they'd probably have support humans (say three or four) doing things like spotting, comms, and other things that a tank crew would do, including manning machine guns to defend against human attacks on the giants.