So, I only just finished the "Shiel", where at the end you get the first elf rune/plans. I find things so far extremely confusing.

Is melee the only way to destroy buildings? I usually can't manage to get more than 8 units attacking a building at once. Even then, it takes a minute or so usually. Is it unreasonable to expect titans to win against a few towers?

Which leads me to the "mighty" griffin knight. Am I somehow using it wrong? It seems marginally better than the upgraded paladins I was using. Even level two forest bugs get hit by a whopping 2 damage by the massive thing.

Should I somehow be obtaining to a better worker rune from level 4? The upgraded marksmen are the only thing performing at a level above disappointing.

Well all the titans have a special aura that is very good usually. Titans are not meant for destroying buildings and low level rune does not make a mighty titan. Later in the game they get much better but they are no overly powerful by damage means rather their aura is usually pretty good.

yes melee is the way to take down towers.

Your troop strength gets stronger as you use higher level monument runes. When you use a level 5 elf worker rune instead of a level 4, your troops, workers, titans, etc get stronger. So early maps your troops will be so-so but at higher maps they will be strong enough to be more useful.

A higher level titan is pretty strong, but it isn't invulnerable. It has a lot of health so it won't die fast, and it's always good to keep an eye on it and move it away if its health gets low, they cost a lot to produce and you don't want one dying, better to move it back so it will heal up proper.

For humans, marksmen are great troops, as most "distance archer" type troops are good. They stay safer by not getting into melee, and with large numbers can usually kill stuff before it gets close. Human marksmen, elf wind archers, dwarven defenders with thrown weapon upgrade.

Towers are a major benefit also, often more than troops. Elf and dark elf towers are small and you can pack a bunch in together. Troll stone throwers are very powerful but larger. Other towers give and take. Use towers for farming spawn points with a couple troops to draw foes towards your towers.