I leveled up a fly to level 10 to see how strong it would be, and was seriously dissapointed...

While it got noticeably larger, it barely won a fight with a level 1 archer....not sure if there was a bug involving leveling up flies, but level 10 warlocks at least seem much more useful.

Fly are purely scout unit, not meant to do anything but explore the map

It's always been this way in DK, it's unbalanced true, but realistically even the author never bothered to fix the fly level up table, in fact at a certain level the fly get the vision ability, that doesn't even work in the game

Flies are complete wimps, regardless of level. Other creatures though, you'll really notice the upgrade difference. A level 10 Bile Demon, or a level 10 dragon, now you're talking

So flies are bugged and even keeperFX doesnt fix their level up stats?

Is there a wiki or something with the creature stats btw? I know they are in the game files but its in a really hard to read format.

They're not bugged as such, but a level 10 fly really is only better at scouting, seeing as that's their primary task. if you level up a fighter, it will get better at fighting, level up a warlock he gets better at casting spells. Level up a fly, and he probably flies faster and might see further, but he's still crap at fighting.

What were you expecting from a lowly Fly, even a large one? ;)
Creatures have different "jobs" - some are scouts, some are just weak cannon fodder for the early training levels, rarely to be used again after a while, some are hard strikers, some are doorstops.

Lvl 10 dragons are hard to kill, but also slow to kill tougher creatures (doorstops...). Lvl 10 Sorcerer's (Warlocks) have better/more spells, but are still glass cannons and easily killed in melee.

I don't have a list of stats per each level, but this page has the lvl 1 and lvl 10 stats.
http://www.thanatosrealms.com/dk/creatures

Flies just plain suck. Boot them out of your dungeon unless there's a large lava-field or lake that needs exploring. Beetles aren't much better, and once you reach the second mission you'll have demon spawn and can purge all those useless insects. Fortunately, you will get the Spider as a minion before long, who will deal with any insect infestation in your dungeon quickly and safely; unlike most of the minion-on-minion in-fighting, there's no real risk of the victor getting injured.

Some tips...

If you want to pack loads of damage, lightning is a very good way to take mobs out.

Use loads of Skeletons as they are cheap to train and on payday, and only get lightning at level 10 but they are fast to train up. More of a ranged attacker than close up. But the damage in groups of them is very powerful.

If you want a tank, and fair bit of damage but very good survivability, try Vampires, though expensive to own allot of them if the map doesn't have much gold/gems.

Mistress's have lightning, there damage is very good like skeletons, and they get heal spell and teleportation like vampires. But you must keep them well fed as if they go hungry they won't attack the enemy and just walk away.

Orcs are great for close up combat and really can knock out the enemy, especially in groups they are fast attackers and pack a hard attack to knock them out dead, and cheap on payday.

Some maps it might be ok to use Bile Demons cause of their gas.

Dragons more of a tanky build and there flames can really do allot of fast attack damage expecially in possession mode. They are slow though, and are at risk of getting lightning to death fast.

Lightning traps can be quite good for damage, if you lead them through a walkway of lightning.