Did you miss the concept of human reaction time? Iron Man (to extrapolate from top end firearms competitors) can probably fire in .40 to .50 seconds from the bell. Can Dumbeldore teleport that quickly. We don't have a timer on his teleport feats from the bell. Ding and how long to disappear? If he can beat that time, then we can discuss the relative effectiveness of their output when he reappears.
If D. has to think about it, he gets blasted.
Can Dumbledore shield himself, as he did against the glass shards?
About reaction time, here's the deal yet again.
Fast human choice RT is about 250 milliseconds. That's see a light, press a button. That includes seeing it and issuing a motor command. A trained shooter can fire a shot with gun drawn in about 300 to 400 milliseconds. So I'll give Tony whom I assume is in good health and practices these times and fires at the bell.
Yes, Dumble can teleport someplace in the arena? Can he beat Tony's time to initiate the teleport? If not, he gets hit with high tech weaponry. Do we have timing feats for him to react to a signal and can he resist the weapons?
Timing in a fight is everything.
A casual search suggests that with the latest suits, Tony just thinks a command. So, it would be an empirical question as to who is fastest with an action.
Thus, we don't know who can act with what speed and if they can block each other. I guess we don't want to do Dumbledore vs. Vegeta. Just kidding.
"Latest suits"? Iron Man has been controlling the armor with brainwaves since at least the seventies. The only armor I can recall that didn't do that is the Warren Ellis armor. He ignored continuity so he could give him the power right back via Extremis.
There's probably a limit to Ironman's ammunition so can Dumbledore keep away long enough?
While his rockets are limited, his energy weapons aren't limited in real time.