The only advise i could give is: Keep on trying. There is no way to tell what you are doing "wrong", if there is somethin. On hard you might not get the right drops early on. You might trade to many pots for Waters. You might place it in bad spots (the towers) or just using weak towers. The onyl advise with the provided informations is: Keep on trying and find your own way that works. And it might not be an good idea, if you try to go from starting towers to yellow/purple towers. Electric chair and the like can work wonders if placed on a good spot. Mallets are good items early on when you rarely crit anyways... Dont skip too much. Skip enough if you play with the Electric Chair. It always hits the first target when its shooting, if you time it good you can catch some leaks... Towers with abilitys (Shadow, Scarface, Jilly, Spider, Acolyte... not meaning splash like Dande or Balu, but burning like Solara) deal less dmg on build up. You might have to go for 2 Carrys, one for mass enemys one for single targets rather early. The rule with "one tower for first 10 rounds and then 1-2 support towers" goes overboard if your not lucky with items/pots/waves. You might want to restart a few times until you get an early wave you can take down and got "Wisdom" or 2 in a row. Good EXP boost. 2 Wisdom waves and you might end out Lvl 5-6 on wave 3. For more specific Advise you might want to share some informations about your buildup and items, which map, which "races", which game lenght, what intend (just go until bonus rounds or you want a top score?)
Turns out I was just expecting it to go as smooth as it was going on easy and normal, I am doing better now, starting to use shadow again after all this time.
Shadow is a big help in hard mode because its stacks lower the effect of bad drops. Anyways, in my experience, you just don't make it in every hard game. Once you've reached wave 150.200 however, it should start to become much smoother. Also, you should absolutely get the hp bonus. I usually lose at least 150% even in good games. (In my current game, which is at 2200 seconds bonus round right now and still running, i only had 11% left for the most time of the run.)
I got slaughtered the last two attempts using the Hard mode and the new HardMode wizard. Around wave 60 bad things start to happen. Your main doesnt level up as fast, your potion effects dont level as fast, your itemization only slowly improves, and support tower damage becomes negligable. At around 80-100 you start getting Unique towers like the MultiCrit booster, XP booster, as well as stuff like Tear of the Gods, Excaliber, etc. These all hugely improve your damage. But that middle ground until then is just really difficult. Even with Shadow and what seemed like a good start, Ki got killed.