If you want lifetap and your merc to survive ubers:
a5 frenzy
2x LW
crushing blow machine that supports you with lifetap and lvl 34 might, can survive ubers
you can also borrow one of his LW to proc fade
For general pvm:
a2 might reaper or pride
I don't like relying on merc to proc decrep when I play a shifter without the ability to reposition merc with tele
so personally I either self-wield reaper, or use it on swap instead of cta, and give pride to merc instead
but for HC, I would self-wield a eth zoded reaper
because it's cheap, reliable, and always proc decrep which is also very good defensively