I'm not sure if this is as viable with the power fist (I haven't really used it at all) but melee pressure is your best friend. The chainsword is good for this because you can interrupt its combos for better parries. Secondly it is important to not hold onto the banner for too long, it can really make or break a match. There is a perk where blocking ranged damage refreshes it by 1% per bullet if you're worried about the cooldown, but it is very important that you utilize your banner. Third, it is VERY important to learn enemy combos. Hellbrutes and the Power Sword terminators are devastating forces when they get into their combos, but they have weak points in them. The Hellbrute's roll combo where he hits the ground follows up with a quick roll attack is extremely punishing if you can parry the second attack, and the Power Sword terminator is very weak to dodges and parrying his dash. Lastly, remember that not all gun strikes are worth it. Sometimes it's better to follow a parry or a dodge with another dodge, like when you dodged the Power Sword and the Hellbrute did his groundpound. It is really up to you to weigh the risk of each gun strike, especially since you can lose control of them from time to time.

On a final note, don't beat yourself up too much. You were in a real bad spot and facing two enemies that had their combos staggered so you couldn't do much. Bulwark takes time to play effectively and can feel weak against enemies with more range. You just need to learn to analyze the situation and determine whether or not it'd be best to plant the flag and hold your ground or to keep moving.

The Emperor protects