"but this does give the Cult an upper hand, as with all the infighting that goes on in Drakken society they end up killing any of their own who are over a certain age, so most Drakken are probably between 50-300 years old" Except the period you have stated here is actually TOO BIG for optimum military service if we are saying 70-300 is the age of military service and iirc it was said that Drakken remain physically fit for an extremely long time. So we're talking 14-60 in Gem years if we're going to compare (Optimum service in humans be 16-45, which the gems are far closer to) so if the Drakken are fully capable of serving in this range them having lost many of the upper age boundary doesn't really mean much. Also, just because they are mentally young doesn't mean they don't remember their experiences and learn from them. A 200 year old Drakken warrior may have the mental ability of a 40 year old Gem, but he presumably has 120 years or so of experience still.
And I still contend that its silly; The Drakken would surely extinct if it took that long for them to mature given that they live in a pretty awful place. They definitely do not have the time to spend 90 years maturing if they live in a hostile homeland; Honestly, the concept of a 90 year development would still be way out there for even the Gems who live in a relatively safe place because its just an absurdly long time. They might be long lived but that doesn't mean there isn't any pressure to mature quickly. They might be able to live 500 years, but if we consider that their homeland is a pretty hostile place without all that much in the way of food, they could easily fail to make it all that long in nature; in fact, their long life is probably just a byproduct of their evolution, a number that in nature they would never reach anyway. You're basically saying that they'd spend the vast majority of their life in their child stages (potentially not even making it out before disease, famine or violence took them. Basically think of all the things which can go wrong and kill someone in a humans full lifetime of 80+ except for old age these can all still go wrong and kill a Drakken in this period, except that instead of making it just 15 or so years before they can start reproducing, they instead have to make it 70 years. It just wouldn't work out all that well.) and thus unable to breed, couple that with their genetics already being weighted in favour of males and not females and such a crippling development cycle doesn't make it fair on the gems, it instead raises the question of why the Drakken are seemingly designed to go extinct; Evolution produces species which are good at passing along their genes. There is a reason you don't see many species who take 70 years to mature here on Earth; They wouldn't be very good at the whole procreating business.
"It sort of seems like Gems would have at least better technique" Why? Unlike the Drakken, they don't care about war. It seems very, very unlikely that they have given much thought towards military applications of their abilities and more likely that their knowledge of their abilities is far more domestic. This isn't better technique on either side, its different usage.
As for why would they use their abilities because they're abilities would surely be extremely satisfying for some of them and would provide answers to tactical problems that would otherwise be difficult to solve; Imagine how differently ancient battles would have gone if one side was able to crack open the earth, or drown their foes in a flood, or bathe them in fire etc.
As an aside, sword fights often ended within a few seconds of starting and were typically in favour of the man with the most experience, training and the best equipment. A 39 year old peasant was chicken feed to a newly ordained knight of 22; This is basically the equivalent of the average Gem versus the average Drakken. Even if the Drakken is younger mentally younger, its quite probable that he's still far better at killing things.
Also, medieval warfare was never fair; If you look at the casualty figures for great battles, you'll see one side with relatively few loses and then the other side with have enough dead to build a dam out of. Look at Agincourt; 112 (at least that is then confirmed amount. Modern historians believe it was probably around 450 in total) dead English men, 7000-10000 dead French, with 1500 noblemen taken prisoner. (these figures can not be entirely trusted, naturally; There was a great deal of sensationalism in the English records and a great deal more damage control on the French side, but these are the modern figures.) Along with a number of important figures who held high offices pertaining the governance of France being killed.
As an aside, the majority of the English army in that battle were peasants with longbows, versus an army which may have outnumbered them as much as 6 to 1, with 10000 knights and thousand of infantry, crossbowmen and archers. For all intents and purposes, the battle was stacked against the English as heavily as the invasion is stacked against the Gems, yet they absolutely hammered the French. Why? Well, the same way the Gems can win this war. The French knights got arrogant, they attacked too aggressively and got butchered by the English who played off their own strengths.
