Due to the long sales cycle, I wouldn't say that there is a specific channel that works (or a precise combination of them).
What you need for sure is:
- 1.
Event attendance. You need to be where these people are and meet with them multiple times to even get on the vendor list. Bonus, once you have a list of good leads, I recommend setting up your own events, where you control the topics and speakers.
- 2.
Thought leadership. Your company's senior management should be speaking at events, appearing in the media, posting to their networks on LinkedIn. The personal network trump business ones for these type of clients that you are targeting.
- 3.
ABM & social listening. What are the top 50-100 companies you want to work with, how does the decision making unit look like, who is part of them, what are they talking about? These will then feed into #1 and #2 I was mentioning above.
- 4.
Partnerships. Identify communities & leaders from the industry that they follow and see how you can build something valuable for the communities that gets you extra exposure for your target audience.
I find that a good way to engage people is by doing a research on a topic they care about - they'll be more willing to answer and it's a foot in the door for you to start that relationship building.
And as Manuel H. was saying above, good quality content, that is also shared through a network of partnerships and targeted ads.
Hope this helps! Good luck!