Strategies
Juggernaut offers the most utility as an assist. With no real options for dealing with runaway a lot of matchups are going to feel like a struggle. Juggernaut is slow, easy to cross-up and combo, uses meter far better than he builds it, everything he does is relatively unsafe and expensive to protect, and he has no super armor in the air. However, Juggernaut has one thing that changes him from an annoyance into a problem, the Power-Up glitch.
Your first goal with Juggernaut is to activate his Power-Up glitch as soon as possible. The fewer bars of meter the opponent has when you change to another character, the harder it will be for them to punish you, especially if you change to a character with high vitality. Once glitched, your assists offers game-changing amounts of damage and your Headcrush threatens to one-shot many popular assists.
Your second goal with Juggernaut is to land his assist as many times as possible. Juggernaut has a lot of health but he's not invincible, his large size means it's going to be harder for him to avoid damage so be ready to protect him when your opponent retaliates. A glitched Juggernaut Punch can kill any assist in 3-5 hits instead of 5-7 hits which makes counter-calling your opponent's assists a powerful, meterless threat.
Your third goal with Juggernaut is to land his Headcrush as many times as possible. Headcrush is pretty easy to combo into from most of Juggernaut's normal buttons, but landing that initial hit can sometimes be difficult. The best buttons to approach an opponent with are probably going to be jumping 
and jumping 
. A throw when near the corner often turns into a guaranteed Headcrush and a potential touch of death with the right assist. The THC Headcrush is unmashable but has a faster startup than a normal Headcrush allowing for monstrous whiff punishes. A DHC from a better point character into Headcrush can turn a stray hit into a potentially favorable match.
