Does the hp of enemy units go up with difficulty level? I dont understand how some enemy units have like 300 hp and only 3 units, when my guys have like 30 hp and 4 units. Seems really odd, dont quite understand.
Experience levels.
Experienced/veteran units not only have more HP then regulars when you create them, they also gain more HP on level-up. The difference will be multiplied by the number of levels the unit has, and then again multiplied by the number of figures in the unit. I can imagine with high enough level, original experience at creation and figures in squad 300 HP would be reachable. However normally i see them in the 120-150 range.
[EDIT] it also heavily depends on the base HP of the figure. A darkling warrior has 15 base hp(instead of the normal 5 of regular unit), so a squad of veteran warriors who are level 5 would have very high hitpoints.