WOD is your friend. You want to try to bait your opponent into using wiz' effect on things that aren't as useful for you while depleting their hand so that you can use what you really need when it counts. Also, you don't always want to kill all of their monsters each time. If you WODed them, try to build up your team a little more for the kill instead of letting them get another donpa.
Try to activate your onslaught before they summon wiz. Wiz negation effect will not send onslaught to grave if it is already face up. If they manage to summon wiz before you have your onslaught, bait wiz's negation before activating onslaught. You can also use chalice to negate wiz. Prioritize banishing wiz/donpa to make sure your onslaught is always alive.