I've found the easiest way to beat 30-Difficulty is to use the Prince. Your king becomes mostly unkillable and then its just a matter of turtling and protecting your prince. Portal mage or decoy can also be used to great effect to protect the prince. And you can also use units like portal mage or blade dancer to get free kills, while you move your king towards theirs. I've gotten to Infinity-20 on 30-Difficulty this way.

Thanks to both. Coincidentally, I am currently doing a diff 10 run at infinity 60 with two blade masters, a reaper and a prince and am pretty much unkillable since they nearly let the blade masters come close enough to chain the enemy general and items/reaper take care of sticky situations. I'll try and build up a similar army on diff 30

i beat dif 30

use a prince and attcking pieces

if you are just looking for the achievement without the challenge, you can turn the difficulty slider in settings down to 0, as you don't need to be at 100 difficulty to get the 30 difficulty, the enemy will then pretty much just walk into your path and it becomes very easy, i did it this way to get 100% achievement and then went back at a gradually higher difficulty slider, im up to 40 % so far and enjoying the challenge, i hope this helps!

Another alternative solution, you can get the achievement if you activate the relic that gives random pieces after every kill

The game does an extremely poor job of explaining what this achievement even is. I've completed the game at 100% difficulty and don't have the achievement. I've tried setting the difficulty to 30%, also no achievement. The max amount of boon points is 9, so what gives?

32 levels into infinity mode and still no achievement either. Uninstalled.