Wow. Not often one sees such a blatant request, so I will start from the beginning:
No pentester should ever agree to DoS attacks with the level of knowledge and outage you have and are discussing. It is both highly dangerous and mildly... humm... Immature. You are causing potentially millions of dollars of damage, which needs to be covered by insurance and all the rest.
Not to mention you really, REALLY need a signed statement saying that you can do this. You wouldn't catch me on one of these assignments without a couple of lawyers ensuring it was watertight - and even then it would be a stretch.
If the owner is onsite and onboard, have him walk into any comms cabinet he has and take out any thin orange cables with square (not rectangular) connectors. As well as anything plugged into a switch at ports 1, 2, and 41, 42 (assuming a 42 port switch).
Doing this lets you perform mass DoS conditions which are relatively easy to correct (all we did is remove the fiber connectors and the common switch uplink ports), but will cause panic and problems for the IT team.
In theory, purely arpspoofing the entire network with // // would cause massive issues, but it depends on monitoring tools, 802.1x, port security, vlan's and a few other techs. You should not do it though.
Resolving the spoofing issue is fairly easy though - crack the network in half by the core switches - one side or the other will still be being poisoned, then crack that half in half, and so on. Mathematically you should be able to discover exactly who the perpetrator is (by switch) in 7 or less splits.