Naudet, I think maybe being unpredictable is better than being 'aggressive' in the pure sense of the word. There are different levels of aggression. For instance, lets say you spot a lone Spitfire about 5 thousand feet below you. Of course, you dive on it, and you really hope it doesnt see you so you can get an easy kill

. Unfortunately, he does see you, and breaks into your attack. The least aggressive option you have here is to simply zoom off and go looking for someone with less SA. The most aggressive option is to simply accept the HO and hope you have better aim than he does (this one I very rarely take

). Or, you can do a short zoom, watch out the back of your canopy to see what he does, and then attack again.
Lets say for our purposes that he merely continues flying straight after he breaks into your first attack. You zoom to set up for another attack. You come out in a fairly steep dive coming from his 6. Lets say he does a hard break turn to the left at a range of 600 yards. Again, you have a few options here. You can just abort the pass and zoom up again (like you did the first time). You can go for the shot anyway and possibly expose yourself to a snapshot from the Spitfire when you overshoot. The third option is best, and what I normally do. I'll pull up like I'm zooming, but I'll keep a visual on the Spitfire. If he tries to barrel roll or turn to his original course, I'll simply come right back down after him after my 'short' zoom. If he stays in his break turn, I'll climb and then do a 'high yo-yo' (basically you pull up into a zoom, then roll so the top of your head is pointed to where the spitfire is going to go [the technical term is 'lift vector], then cut inside of his turn and go for the shot.
The nice thing about being in a Dora against a Spitfire is you can be VERY aggresive, even to the point of foolhardiness. As long as you stay above 300 mph, the Spitfire can't touch you (assuming you started off in the attacking position). You can turn with him if he tries to break turn out of your way, if you miss the shot just roll wings-level and disengage, he can't do a damn thing about it.