author is seriously underutilizing his powers, he could easily very safely tunnel all the way back, he could hollow out a massive cave under enemy forts then just use dynamite to blow up supporting pillars and collapse the fort into a hundreds of meters deep pit, or do like the allies did in Messines, dig in under the enemy front, in this case, under their forts, make a large cavern, fill it with dynamite, then boom, massive crater.
i would 100% do just that, screw us taking any risk of injury or loss, do its safely like this and utterly obliterate the enemy forts and their morale, fill in the crater and make a new better fort with 1 meter thick steel walls and another 1-2 meters of brick walls behind that, place the most suitable powerfull modern weapons he can on the walls, a truly impenetrable fortress by their worlds standards.
its a very common issue when authors give characters to OP powers with little to no limitations and balansing, they just illogically and stupidly have the MC barely use his powers for the sake of plot.