There can be two ways to deal with an enemy who's hunkered down into their final corner: both start with claiming all the resources on the map, as well as making yourself radar for vision. The first way is the rush way, where you mass the army you have and overrun their position. Sometimes you already have enough units on the map and by grouping them all together you can beat the infirior army before you even finish building up the entire map's worth of economy. Other times, you need to use the army you have to contain them in that corner while you build up overwhelming numbers. But it sounds like you're having problems with incredibly fortified enemy positions, where even just attack moving a large army in is not an option.
That leads to the second way: attack from range. Most factories will have an artillery unit that can out range the usual defensive structures(less than 1000 metal), like impaler(rover) or sling(cloaky). The cleanest kill is the one where you don't get hit, so stand an army of assaults and riots out of range of their defenses and clear them out with your artillery. You may want to morph a light radar tower into a sparrow scout plane to scout the lay of the buildings, or go all the way and build an air factory for Owl, the best scout in the game. But say your problem is an enemy lucifer, which I have seen in low ranked 1v1 games. A lucifer will kill just about any lone unit you send in it's range, no matter what. If the enemy has a very strong weapon like this, you need to scout their base with maybe a couple scout planes and see if there's a weakness that's undefended you can strike. And one of the greatest weaknesses is no riots. Lucifer will kill any individual unit, but cannot kill an entire army on it's own. If a Lucifer costs 2000 metal, that's 30 glaives. If you capture the map for the metal resources, you should easily be able to double that, 60 glaives(or equivalent of other raiders) and attack the enemy base from all directions in a reverse fan. If there's no other enemy riots, superior raider dps and numbers will defeat everything.
You also have counter- porc and silos. Use an army of constructors to build some defences in the mid, and build a missile silo(special tab) in range of a couple key enemy structues of lucifer weight, and hit them with tacnukes before rolling an army in.
Back to an air factory, you can build a giant bomber fleet and start picking off enemy targets. Look into different forms of the
ForceFire command for bomber control. Use the Owl to spot good targets.
