This is where improved awareness comes into play; and usage of flashbangs.
All Scions look distinctly different to one another; and there’s obviously the vocal cues too, so with some experience you can work out what types of Scions are on the field. If you have a good headset you should be able to hear the vocal cues and what direction they’re coming from etc. Even hearing the footsteps of Scions approach from whatever direction helps.
Flashbangs are great for stunning heavies - this should be a priority for Boomers and Mulchers (and to a slightly lesser extent Salvos as their firing pattern is a bit more predictable but the rockets still pack a fairly big punch). Other heavies like Overkill DR1’s and Dropshots are more easily attacked without needing a flashbang.
Just to add, the gameplay loop you should be aiming for is a little bit like this:
(1) Do heavy damage on enemies to build up your ultimate - a Breaker Mace obviously helps with this on later waves and especially on heavy enemies
(2) Use Electro-blade with Shockchain for maximum effectiveness
(3) As the ultimate runs out, retreat to safety
(4) Rinse and repeat
With step 1. you need to always bear in mind that ultimate recharge is largely tied to your damage output nowadays. The more damage you dish out, the more your ultimate recharge metre is reduced. So in terms of game mechanics are concerned, the more health the enemy has, the more you can recharge your ultimate - you’ll notice that once the progressive Horde buffs kick in of 2x health, your ultimate recharges fully from killing fewer enemies (and also faster) as they individually have more health per unit.
What I’ve noticed good Blademasters do, is to target heavies, and use the Breaker Mace to deliver enough damage, before retreating. 2-3 swipes or a single ground pound is usually enough damage on master difficulty to do enough bleed to kill them a couple of seconds later. Heavies obviously have more health, so killing a couple of heavies will knock off a big chunk off of your recharge metre.
Once your ultimate is ready, switch to another gun (so you melee with your knife) and then activate it and go nuts.