Sadly, the most accurate words I can find to describe Diehard Dungeon are 'tedious' and 'clunky', which is a shame because this could have been a very fun game.
The second descriptor, clunky, is the easier to tackle so I'll go with that first. The controls in Diehard Dungeon are not particularly responsive and the combat mechanics make things awkward. Using a controller does help with some of the problems but even so, moving and turning in combat is generally bad.
As to the 'tedious' part; well, many of the game's mechanics seem to be based around wasting as much of the player's time as possible. Here are some examples of what I am trying to convey:
The majority of rooms can only be exited once a key has been placed in a lock. The key will be hidden somewhere in the room; perhaps in one of the many random objects in the room (every one of which takes multiple strikes to break open), perhaps on one of the hit-sponge enemies. There's no indication as to where the key will be and you will often spend fruitless minutes hacking away at boxes to find the key.
Little pixie type creatures will often appear (most of the time from one of the previously mentioned boxes you are hacking open in search of the key) and steal your companion chest. Well, not properly steal the chest, like take it away to be found later on; they just grab it and run to the other side of the room. You can't collect any treasure without the chest so you can't just leave it behind. What you must do instead is chase the pixies around the room, slashing at them with your sword until you've killed them all and can get your chest back. The problem here is that the pixies are fast and will continually move away from the player character, whilst the character cannot move while swinging his sword. And so a merry game of trying to manouvre the pixies into a corner ensues. And this will happen every few rooms. It's not that difficult to get the chest back, it's just tedious and annoying.
There are other exmples that I won't go into an extended explanation of as they all tend to end with the same reaction of 'Why am I doing this? What's the point?' and a feeling that your time has been wasted.
Other, although much less prominent, annoyances exist in the game, such as: parts of broken objects covering the trigger plates for traps so you can't even see them, enemy attacks that cause you to drop your only melee weapon so you have to go running around to find it again, status effects that make you run in random directions (usually straight at the nearest wall) for an intolerably long time and the fact that the bomb launcher (secondary weapon) is about as effective as launching an attack on a Tiger tank with firecrackers.
The final negatives about Diehard Dungeon are the bland graphics, uninspired dungeon design and the completely 'meh' soundtrack. Overall this is not a game I would recommend unless you have a whole load of time to waste on pointless tedium and absolutely nothing else to play.