The following are some personal opinions I have based on my experience with hero trial.
Even if you have a really weak army and cannot compete in Hero Trial, you should at least do a couple of fights in Hero Trial. Win or lose, you will get 20k gold at end of each season; that's practically free gold.
Currently participation in Hero Trial is still low, so participating players should get a rank higher than their arena rank.
Many players still leave their defense at the default state, which is 5 heroes on row 1 and random troops behind the heroes. Regularly check if you have proper formation.
Many players deliberately use level 8 troops in their defense to lower the power rating so they can bait others to attack. In Hero Trial, heroes do most of the work. Look at what heroes your opponents have before attacking.
A Hero Trial battle can drag on forever if there are Green or Arthur on both sides. If a battle takes more than 5 minutes, the system automatically decides that the defense wins. If the battle drags on a long time and you don't see any possibility of it ending soon, quit the battle and move on.
Hero Trial is about how your heroes work together. Don't just mindless use all your highest-level heroes.
Its better to use heroes with skills that stack. For example, Chione and Edward have speed-related skills and work well together.
These are heroes that wins by grinding out battles
Currently, Arthur is very broken in Hero Trial. Supposedly Arthur should heal human heroes and troops, but currently he also heals non-human heroes such as Aly.
With his shielding ability and health regeneration skill, Arthur can turn a Hero Trial battle into a slugfest.
While Dracula's health steal skill may not be as impressive as Arthur's skill, Dracula can help friendly units live longer and outlast enemy units.
Chione negates enemy offense with her 35% attack speed and movement speed reduction stills.
These are heroes that wins by an overwhelming show of force.
Seondeok can help her army deal a massive damage in the opening (8 seconds). If that is not enough, her health recovery active skill can come in handy.
Aly can crush enemy units with spirit bombs that deal massive damage and also have defense reduction and stun effects.
Selene can help her army deal a massive damage in the opening (up to 7 seconds) while also shield her army from attacks from Seondeok and/or Aly.
Zeus can disorient enemy units with his lightening strikes, making them easy picks for his own army.
If you do not have a high-level Arthur, Dracula, or Seondeok, then Green is usually a must. Green can help heal friendly units. However, be aware that Green has limited healing range and may not be able to heal all your other heroes. Green is also very fragile and can die quickly if the opponent has a burst hero such as Seondeok or Selene.
Have you ever heard about the bug that heroes can't activate their skills? Well, it's probably because the opponent has Ivan. Ivan's silencing skill can prevent nearby enemy heroes from activating their active skills.
Ivan is also a great hero killer and can help you take out opponent's Arthur quickly. Use Ivan on the second or third row, and support him with other melee troops or otherwise Ivan dies first.
Davison and Drake are well-balanced heroes that compliment both blitzkrieg and attrition heroes
Do not underestimate a high-level Kelvins. Kelvins can provide friendly troops shields that rival Arthur's shield.
Moshe provides health and defense boost which can help your other heroes live longer.
Not everyone has a max-level Seondeok, but many players have low or mid-level Seondeok and Drake. The duo can help you devastate the enemy in the opening.