grenades, heavy weapons but not melee thats isn't very affected
The sword is very effective against flood, as well as anything that deals a lot of damage in one go, i.e. the rocket launcher, gravity hammer, fuel rod gun, flamethrower etc. Things like the battle rifle and covenant carbine are good if you can pull off the flood head shot (the infection form imbedded in their chest) but arn't good against pure forms. Anything else, like the assault rifle, smg, pistol etc doesn't really do much damage, especially plasma weapons. Oh and standard melee works ok on lower difficulties, just don't expect it to be as powerful on higher ones.