The earlier Imperial armies have big tercio units. They soak up a lot of damage but also tend to be vulnerable to artillery (as they present a big target) and lines of shallower shot heavy infantry (e.g. Swedish Salvo Foot or Infantry with Light Guns).
In melee combat they do pretty well against most things if not surrounded, disordered, or in some state of cohesion loss (disrupted/fragmented).
The Imperial side also has good Kurassiers. But so do some of their common opponents.
Since any veteran tercios you field are usually going to grab the attention of your enemy (they will either want to avoid or dismantle them), one goal may be to get them into a position that allows you to pressure or fix part of the enemy line.
Flank charging with relatively fresh Kurassiers against enemy units frontally engaged by your Tercios is a sure way to cause them to lose cohesion levels and hopefully even sneak in some secondary charge targets on the pursuit.
Just some thoughts. I think many of the guys here are probably considerably better then I am if you're looking for tips based on a really detailed grasp of the game mechanics though.
I find this is considerably more difficult to implement in practice if the opposing force screens their flanks with light cavalry or unfavorable terrain.
So far, the Early Imperial Army has been difficult to use, mostly because I tend to be outnumbered in terms of units by the Protestants. While a tank-like Tercio is nice, watching it get gobbled up or shot up by numerous smaller units is amusing to say the least. Worse- the Protestant cavalry is so far on par with the Imperial as far as I can tell.
My success with the Imperial army has been using them in places where terrain bottlenecks the enemy. Even on attack scenarios, I've taken to swinging wide around enemy defenses to lure the AI out into a field of my choosing.
I treat the Imperial tercios like battleships. They get escorted by light units which absorb enemy fire for them until they get into attack range. I find arquebusiers or other cavalry work well for this purpose. Three or four Early tercios will mop up the battlefield if you can get them close enough before they take fire damage.
In one game I tried to melee Imperial tercios. Three Salvo infantry broke fighting just one early tercio. They are formidable.
You should take few of them because they absorb such a large part of your manpower but they are battle winners.