trained = -1 to difficulty
specialized = -2 to difficulty
asset can reduce difficulty at most by -2
Edge is a stat pool and at 3 it becomes free 1xEffort
It's not Effort 13. That's how much you need to pay a stat for that Effort.
Edge 6 = Effort 2
3+2+2+2+2+2 is NOT Effort 13 but Stat X cost for Effort 6. If that was the case you could reduce Difficulty 10 (impossible) to trivial or nonexistent most of the time which defeats the purpose of difficulty levels.
Of course. P.87 Numenera Core Rulebook "It costs 3 points from a stat Pool to apply one level of Effort, and it costs 2 additional points for every level thereafter (so it costs 5 points for two levels of Effort, 7 points for three levels of Effort, and so on). A character must spend points from the same stat Pool as the type of task or roll—Might points for a Might roll, Speed points for a Speed roll, or Intellect points for an Intellect roll. Every character has a maximum level of Effort she can apply to a single task."
1 level of Effort decreases Difficulty by 1 step. 3 Edge of certain stat translates to 1 free Effort. So you can take 3 from Edge to get 1 Effort and then another from stat pool. If your Might Edge is 2, then for Might test you take 2 Might from Edge and 1 Might from Pool to have 1 level of Effort and 2 Might from Pool for each thereafter. You can use your Stat Edge only for Difficulty Levels of that stat. If you need Might Test and have Might Edge 0 and Speed Edge 3, you do not gain free effort for that test because it's not the stat associated with the test.
If Might Difficulty Level (pushing 2 ton boulder) is 10 (impossible) then a character who has Effort Cap of 3, 15 Might Pool, Might Edge 2, is specialized in pushing heavy stuff etc and has proper tools that count as an asset, and a friend who can give him a hand (further asset) will do the following. Put a lot of effort (3 levels) but also using edge: 1lvl Effort 2 Might Edge + 1 Might pool, 2 might pool, 2 might pool. DL goes from 10 to 7. He is specialized. DL goes from 7 to 5. Has proper tools and a friend to help (assets -2 to DL at MOST). DL goes from 5 to 3. Roll 9+ on D20 to succeed.
Even then with all that there is only 60% chance of success :) Page 15 Numenera Core Rulebook has it all covered. P.20-21 about Edge, Pool and Effort.