I usually use catapults (both types) as my main ranged units but can magic weilding units be more effective and which ones?

I never use Archers because they are useless, don't do any damage and die too easily, but if i can get Green archers, i love them, how can i reliably get them?

Most ranged units are underpowered compared to melee units of the same tier. There are some exceptions to this, mostly units that can get double shot (Bowmen, Slingers, Horse Archers, Elves), but even then you need to level then up and get them some medals before they're much use. The other major exception is if you're using a commander multiclassed to scout, as they provide a bonus to all ranged units on top of the commander skill that also enhances ranged units (one of these loaded up with crossbowmen for tier 1s can actually do pretty well, as long as your higher level units are mostly melee to protect the crossbowmen).

For higher level ranged units, catapults may seem nice initially but they're ultimately pretty mediocre compared to other tier 3 units (the tier 3 priests are quite good, but for their healing and resurrection abilities, not their ranged attacks). Magicians (tier 3) and Warlocks (tier 4) are decent on the damage side of things (since they also have spells to supplement their ranged attacks), but again against other tier 3 and tier 4 units they're fairly mediocre in terms of raw power. Ultimately melee units will always have a higher potential damage output because in addition to their attack they can also be dealing lots of counter-attack damage each turn (the most effective melee units are those you can park in the midst of the enemy and have them wipe out most things with their counterattacks).

On the lowest two difficulty settings ranged units can seem awfully nice since everything dies to them so fast, but on expert and above you really need a mostly melee army that can both give and take damage in extended fights against larger armies.

It is funny that for the player, everyone says melee based armies are supposed to be more effective but at the same time i find that enemy ranged forces seem to be the more effective opponents in the game, how can this be?

Also it is a bit confusing exactly how to train a Commander for a ranged army.... Specifically, the descriptions of the ranged upgrade options for the Commander that is? Some options seem to suggest increased capacity for the whole army, some are more ambiguous and some seem to suggest it is only applied to the Commander? What exactly is going on here?

Enemy ranged opponents likely seem the most effective to you because they're the ones most likely to kill off your units in what would otherwise be fairly one-sided battles (due to them being able to focus their attacks on your weakest units). Get into a few serious, last-man standing type battles and ranged units will often become the least of your concerns (they'll often be out of both ammo and stamina before the battle is even half over).

As for commanders, all of their upgrades apply to the whole army. Level 3 Ranged Tactics will provide +2 attack and +1 ammo to all ranged units in the army, and if memory serves multiclassing the commander to scout at level 10 will provide an additional +2 or +3 damage to all ranged units in the army. If there are any particular commander skills you're still unsure about name which ones they are and I'll try to clarify.

Use a Commander and at level 10 make him a "Strategist". Then he gets another +2 range attack in addition to the skills for the troops. Even slingers will be a menace.
He will also have the Scout Ranged attack skills available which apply only to the commander.

For a mage 1 or 2 strong ranged units in the army can be pretty useful if you buff them up with all kinds of spells and get them those range medals.

Technically, any living unit would be better than mechanical because they can be resurrected (saving veterans).
Intended use for ranged units - slot filling during clashes with strong opponent at T3-T4 level - both lose most/all veterans, fresh T1 melee unit can't do much at this point, so you fill all slots with ranged units, they can deal damage without being immediately decimated.

Magic weilding units in most cases better than simple ranged damage units, because they bring more on the table:
Shaman has highest damage as fresh unit (at T1 level there are almost no resistance, and many units have 3+ ranged defense)
Sorcerer - Vulnerability&Fear would scary even elephant or strip all armor from tank.
Monk can remove poison and restore stamina - useful for doubleshots.
Priest - resurrection. Removing buffs/debuffs is REALLY useful in pvp.
Mage - mind control. Dominate T4 unit, sacrifice it to summon devil. Table has turned.
Warlock - isn't very good against human opponent, but can be used perfectly to trick AI with countless summons in back row.

As for green archers - elves - you should find their province and ask for quest. Complete their quiest and then you would be able to build their building in the castle.

Thanks all for the info

I have a level 30 Warrior in my current game, there are few enemy he fears; the big catapaults - more deadly even than Dragons! Simply for the fact that they are often hard to get at and he dies before killing them. Having a flying belt would fix this but he doesn't have much good equipment because i have been suffering a very bad case of of repeat disease in this current game.

Repeat disease - continually finding the same equipment over and over, thus preventing finding needed equipment. I think i have found about 10 Dragon gloves so far and not a single other piece of dragon Armour!!!!

The other things deadly to a top level warrior are spiders and slugs, but spiders especially!

I also have a level 20 commander who is doing quite well and I also have a level 15 archer and i have started developing his looting ability since i am starved of equipment in this game.

Interesting thing in this game, my first Hero was the archer, but because he don't want to win any battles, that has hugely slowed his development - but the main reason this happened was because he cannot get a decent bow for so long.