I invoke my necro powers to bring back ivy tact~
I've found on Battlefield (a good neutral for ivy) almost everyone I play tends to drop through the platform first thing, so I've made it a habit to punish their habit (lol) and drop through razor leaf right at the start.
Battlefield is the best PT neutral stage for playing on, but SV is almost as good and allows you to perform the zero switch, so you can switch on your weak pokemon (like ivysaur for most) without taking the punishment.
Ivy tends to approach best backwards, with nair->utilt, nair->bullet seed, or bair as just good spacing.
At the end of bullet seeds, its sometimes worth mixing it up and using ftilt to pressure their landing.
Rather then going for a spike, often its better to go for an instant tether- if they won't make it, just stay on the vine, if they might, release and attack. Instant tether is also useful because it turns you around, so you pop up, hog the ledge, and are set to use bair, your best gimping tool.
Ivy actually has a really great gimp game offstage, provided they don't turn the tables on you. Walling with bairs onstage results in mere chip damage, but offstage it can mean a stock. Use vinewhips to the ledge to regain your jump and continue to wall them.