I'm not sure how enemy spawns work if you're in a multiplayer matchmaking mode but don't actually have teammates with you, so if you're not getting matched with anyone anyway, you might want to try playing in Solo mode to both guarantee lower enemy counts and to enable you to pause the game. Both Interception and Hijack are made a lot easier with fewer enemies. Interception just becomes a game of taking the points the enemy isn't currently at, then reacting to defend points they try to take. For Hijack, you should make sure to change your frame's mods for better shield capacity (Redirection) and maybe recharge speed (Fast Deflection).

Is your issue with those missions that you're struggling with the objectives, or that you're dying, or that you can't kill enemies fast enough, or all of the above? What frame, weapons and mods are you using? Mods are where the majority of your power comes from, but they can only help you so much if the weapon you're using isn't great since they're almost always percentage-based.