I think if you asked this question on this subforum:
viewforum.php?f=248People would be more qualified to help you there, nonetheless I will just rush you through the basics of HELL version (can't really help with the other versions)
A. Army
1. In HELL version Damage types and armor types matter much much more. For example there is slim to no chance to pass level 10 without pierce or at least magic damage. Or you probably won't def level 7 without heavy or siege armor etc
2. Positioning and formation matters much more. Same value can def much better just cause your army position/formation.
3. Composition.Your army should be as balanced as possible between range and melee units <=> tankiness and dps. Altho this really depends on the wave.
3a. If you play HP, use your rerolls wisely. Else you will get screwed on a long game.
B. Modes
1. There are few different Modes in HELL version, such us AC, CC. The default mode (aka -hpgmcb), also sports Champions (sort of wave's miniboss, altho is some modes they are not really that "mini", looking at you pr/cc modes!). For a No Champion mode you need to put NC as submode.
2. Each mode must pretty much played differently. Am still figure out this one since it kinda very new modes but lumbering/wisping and even builds and sends varies from mode to mode.
3. Champions will make you their bitch till you learn to play good enough.
C. Units
1. Units in HELL are different in many aspects, they almost all have different attributes (hp, damage, attack, armor types, regen etc) and many have different skills and even upgrades. You need to update yourself on those changes cause they can make or break a unit for the upcoming wave.
2. I would dare say that HELL units can be divided into early/mid/late/overall ok units and all the combos in between. For example cheap units are usually a must early, and they do help on boss waves too, example2 Meat wagons are good for early game and super late. There are many opinions on the matter, so play and figure it out or wait someone better than me to help on this one.
3. Summons. A few summons have different abilities else well, and some are really anti-units. So you need to keep your eyes open on what your opponents have.
D. Strategies.
1. There are many modes that are currently be played and just as much strategies. Between AC/CC/NC/normal mode and HP/PR/PH/AP(altho ap is almost nonexistant) etc. Between modes the game drastically change. When to lumber, when to send... I really need an entire guide just for this one, and i repeat am not the best at it, so be skeptic with my words
2. When to send... I saw everything. I saw games won at pretty much all levels (except 9 and 23, altho i wouldn't be surprised if someone did it).
3. You need to work closely with your ally and on a 2v2 game (1-2,5-6 slots) you need to exchange auras if you know whats good for you.
E. Other
1. Mercenary is ok if you can't stop leaking otherwise
2. King have 3 more skills than usual. The Presences (you can have 1 of the 2) and Anarchy. Anarchy is exceptionally good at dealing with leaked Champions and the other 2 depend on the situation. IMO Dark is better vs turtles and mediocre leaks and Royal for big leaks. If its a race, dark is always better.
3. Normal king skills are naturally more powerful and require some form of mana management.
4. Builder has 1 more reroll in Hp mode, and also Command Attack, King's Taunt which can manipulate units attack direction.
5. Clog is your friend.
6. Heroes are good in HELL altho kinda expensive depend on mode
7. Stuns, disables, slows will help you greatly vs bosses. Thus IMO Ents are the bomb!
This is based on my opinion and experiences but also of opinions of others, if someone better than me at the game wants to add/correct me go ahead.
