Yes, bonkadoodles is correct, sadly, the older the server gets, the wider the margin becomes. Top players have a much larger arsenal and more fine tuned runes since they have been playing for nearly 2 years. While the lesser players are trying to max out their top 15 and get runes on them, bigger players have 30+ already maxed out and are focusing on making their near perfect runes even more perfect.
I used to be on the lower level of the spectrum, but was high enough when runes were introduced on server 2 and was able to take part in the learning curve of runes. What started from trying to figure out which set and primary was best for runes, soon turned into how to get the tertiaries and secondaries to match the hero's attributes, then finally turned into getting the right tertiary for a hero to max value. While a smaller player is trying to magic penetration on drac, a bigger player is running a magic penetration drac with a bonus 30%+ piercing damage set up so that drac's impale skill one-shots the enemy team.
Finally, with an arsenal of 30+ heroes, bigger players are able to find many more counters for a line as well as hero synergy. Smaller players are just learning that dark horse is good because he can shield heroes at the start of battle while the bigger players are running brozerker with dark horse so that shields are bigger and longer lasting, not to mention the 5* shielding runes on dark horse.
Most guilds will have at least one member who is able to contend with challenger players, so communicating with that member or members to help you get into challenger will benefit you greatly. If you are not in a guild with someone who can get into challenger, then you have the choice to search for a guild who does have a member or two to help you.
On another note, In platinum I of coliseum and fight pit you will get a max of 150 diamonds in top 5. In challenger coliseum you will get at least 250 diamonds; since there are 100 positions fight pit, however, you can get as low as 150 diamonds if placed lower than 50th place at rewards time. Just remember to get into challenger as close to the rewards deadline as you can. Coliseum isn't much to worry about if your 30th or better within an hour of rewards time, but fight pit gets pretty hectic and you could be knocked from 20th out of fight pit within an hour.
The best advice I can give you is to keep grinding away, learning the game mechanics, and pray that they don't release a level cap and rarity level frequently. As long as your top 15 to top 20 keep at the top of the levels and rarities, you will have a slight chance to get into challenger on your own. It is up to you to look to your superiors within your guild to help you get into challenger as well as learning what heroes, runes, and teams are the best to use.
Good luck and happy hunting