Get your session key that's embedded in the HTML body after you log in. Be careful with this key since it has no expiry and can impersonate you as if you were connected via the default client, including undocumented API access. Find some application that can connect to the Slack RTM API and provide a chat TUI, using the session key you extracted.
I know there are a few out there, but can't personally recommend one.
https://github.com/42wim/matterircd for example
You can use your favorite irc client to connect with the ircd to talk to slack (of course it has almost the same limited irc issues)
Tell me exactly how use slack, directly (without bridges and stuff) without using the official client.