Sharing my strategies
I think the most exciting part of this game is how you can freely choose a wide range of skills from all classes and be able to create successful hybrids. In fact I don't think it would be wise to just stick to one class of abilities.
I've tried Gladiator/Warder hybrid, Arcance/Gladiator hybrid, and pure gladiator. The first two were great successes, but pure gladiator somehow just doesn't work for me. Maybe it's because I've always preferred range attacks.
The thing with this game is, with ranged attacks like bow or wand, you can train endlessly and only suffer minimal hp reduction on death gates. If you keep attacking a death gate, 3 waves will eventually come out. First 2 waves are 2 monsters ea, last wave is 3 of a kind.
After the last wave just leave the screen and go back and the death gate hp is recharged, and you can repeat the above process again. (Just stick to the side of the screen so you are 2 steps away from leaving the screen and coming back.) This training is extremely fast, unfortunately it doesn't work for pure gladiators and shows how you need to spend A LOT MORE health potions if you want to train for close ranged combat.
The reason why I always hybrid a ranged class with the gladiator class is because the gladiator class has some abilities which I find immensely useful. I usually max out fortify and recovery. After I max those out I add 2 points to strength and I get Aftermath as well. (Aftermath in Gladiator class is the same as Malicious Intent on Arcane class, and yes you can get both.)
For warders, when I first played I didn't really try out the skills because I found that if I put a lot into increasing agility (Spirit of the Grove), and put on high agility gear, I can get up to 70% or more chance of critical shot, which is better than the use of any skill.
However if you want more gameplay I would suggest Nature's Grasp, Nature's Recovery, Stalker's Fury, and Blessing of the Wind. I haven't tested out those extensively, but high agility (critical %) + stalker's fury + explosive shot can easily take out half the hp of some of the hardest monsters in 1 shot. There may be better combos but as I said I didn't really experiment on the Warder class, I overpowered the game too soon with that class.
For Arcane class, I basically maxed the same skills on Gladiator class as before, then I put everything into increasing focus after getting Dazzle, Mind Race and Malicious Intent.
When doing shadow void death gates training I've found a good way to reduce damange taken to 0 using Dazzle and autocast even at an early stage. You have to master the use of hotkeys though. When you enter the screen, press space then 1. This will make you autoattack the death gates.
Put dazzle in slot 2. Once monsters come out press space to switch to a shadow void and click 2 immediately. Then click space immediately once again and click 1 to autoattack. You can stack 2 attacks instantly on the second shadow void, which would kill it almost instantly. If there's a third shadow void there's still enough time for you to finish it off before the Dazzle cools down on the first one.
With any luck you'll critical once or twice and be able to suffer no damage at all when training.
Hope you guys find this guide useful!