Finding a full stop to Venusaur in Sun is almost impossible, you either check it spamming priority (and hoping it doesn't gain all the HP back by Giga Draining) or having Heatran and ensuring it doesn't carry EQ / your Air Balloon is intact. Heatran admittedly is pretty meh on a team like yours, while the meta is infested by Genesect + Dugtrio + Torn-T + whatever VoltTurner. However, if Venusaur is not carrying EQ and the Sleep Clause is activated, Chandelure does already a great job at checking Venusaur: it resists everything and can KO back with ease. However, for it to reliably do its work it requires a Spinner, which is quite hard to stick into your team. I don't really have any ideas on how to solve your problem, just wanted to point out that if you manage somewhat to keep SR off the field you're probably not that bad against Venusaur anyway. At least, better than a good 70% of my teams...
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