From my experience making the Orcs:
You aren't really going to be able to make opponents that can stand up to dwarves in toe-to-toe combat, since the invaders will always be unskilled relative to even vaguely trained dwarves. I found adding [NOPAIN] helped a lot, and probably adding [NOBLEED], [NONAUSIA], [NOSTUN],and so on would also help, since they can take hits and keep fighting. [trAPAVOID] is practically required, and lowering speed to elvish levels helps too, to simulate a few extra levels of agility than they actually have. DAMBLOCK and natural attacks can also be used to buff up wrestlers a bit, which are normally just cannon fodder. Adding in large/competent/buildingdestroying mounts can work well too, since they will effectively double the number of combatants.
Making crossbow bolts do less damage, and not be piercing, is probably a good idea. Multiple entities of the same type, active in different seasons, will increase the number of sieges/ambushes/etc. that you get, which is also good. Autumn active entities need special care, since they can appear with the caravan. Fast siegers or ambushers can easily take out the dwarven caravan or liason, which makes everything more difficult (especially with mandates for imported things).
On entity design, I'm currently working on a flying skulking race, the hope being that flying itemthieves will be able to grab a lot of stuff that would otherwise be difficult, and flying ranged ambushes would be a pain to deal with. It might be a path to look into, as an alternative/addition to the lizardmen. Stealth, in general, is good, since it normally results in at least one kill per ambush, and if they aren't as weak as kobolds it can result in kills from thieves too.
Another thing to carefully consider is what of the stuff that you give to your opponents the dwarves can use. Mythril, for example, could be a problem if the dwarves can melt it down and use it against the next attack. I'd suggest buffed iron weapons/armor (or even massively-buffed copper weapons and armor) so that they provide damage/protection at the levels you want but don't melt well.
Add more, and more deadly wild animals. They show up randomly and constantly, and if most of the wildlife on the map wants to kill and eat your dwarves, life becomes harder. Look at Nist Akath in the early years, or Blindquest now: a few aggressive animals can seal off a fort early and prevent migration. Give Elephants back their aggressiveness, make swarms of giant army ants, add in wandering non-megabeast demons, make above-ground giant cave spiders, or just buff the animals that we already have, like wolves and ogres.
On a non-opponent front, consider increasing the growtimes of plants to make starvation and thirst more of a concern, as well as limiting the growing seasons. Adding many rare materials will also increase the likelihood of having a noble who will mandate a material that can only be imported, increasing reliance on the outside world. Giving dwarves the [SLOWLEARNER] tag, and increasing their speed (to 1200 or so) can also help prevent the rapid development of super-legendary dwarves. For extra fun, remove [liKES_FIGHTING] so that the effects of combat aren't buffered by happy thoughts.
And, at some level, the player will need to limit themselves as well. A fortress can just seal itself up (barring mandates + tantrum spiral), and spike trap alternators aren't that hard to set up (and they kill everything).