Would help a lot if you gave some names, but I think your centaur guys have like no armour on, so just point an organ gun at em and watch them melt.
Chaos Dwarf infantry is insane, with ensorcelled HW's, some CD 4+ armour and shields.
Worthless goblins are hobgoblins, just as normal goblins, they are complete shit at combat. Should be obliterated by any dwarf unit with relative ease.
I'm guessing his massive mortar is a hellcannon. Shoots a pretty good small template blast, probably really good against dwarves. In combat this thing is like a daemon prince (WS high, S5, T6, has like 5-6 wounds), also in Warriors of Chaos book it has a 5+ daemon ward save.
Well first of all, I think empire troops will go before chaos dwarves, as far as initiative is considered. So try take advantage of that. I think about anything will deal with hobgoblins, though you should use state troops or something, knights will take too long to grind trough them.
Basically shoot the scary stuff, bring cannons and a helblaster. You can go in combat with the hobgoblins, they won't really put up much of a fight.
You definitely want to bring demigryphs, you can engage his Castellan + Infernal Guard with demis. Sure, the Castellan will probably get trough our armour with a great weapon or something, but the Infernal Guard shouldn't really bother the demis too much. Maybe buff your demigryphs with like Wyssans (+1S,+1T) or debuff the Infernal Guard with something like Soulblight (-1S,-1T).
Try to outshoot his war machines with your own cannons, if you fail, you're gonna have a rough time. If you kill his war machines his infantry is just above average (Not as good as Warriors of Chaos, but not bad either) and they only have M3. If you bring horses you can take advantage of this.
Best of luck!