Make it a Toggle Tap/Hold. If the stats are problematic as they are now, simply just half the damage reduction % and damage bonus for Helminth, but keep them scalable with Power Strength to incentivize players to build for it - so that people aren't turning their warframes into DPS Tanks with no investment.
Yeah I get it, 100% damage bonus ain't anything to write home about and 37.5% damage reduction isn't gonna protect you from very much - but I feel that's a good starting point for an ability when not factoring in mods and other abilities.
Highly advised against f*cking with the duration however, 25 seconds already isn't very long to begin with.