As the staff's resident expert on Grass-types...
Dhelmise is cool but had the nasty predicament of being a hard-to-find late-game mon in a game with Decidueye in it. It's stats don't help it much either. He's a slow, bulky, and physically oriented Ghost/Grass (You know, like Decidueye). Considering he has access to only the more unreliable of the two decent physical Grass attacks (Power Whip) and there just isn't a good Physical Ghost attack that isn't signature to someone else (Spirit Shackle, Shadow Force, Shadow Bone), his offensive output is limited simply because of its types low movepool. Defensively, Ghost/Grass is blatantly awful - Ghost only covers for one of Grass's many weaknesses while stacking on two extra weaknesses of its own in exchange for two immunities.
That said, it is unique in that it essentially has 3 types as far as offense is concerned - the Ability gives it STAB on Steel moves in addition to its natural Grass and Ghost STABs. Combined with his high Attack and low Speed, this means he can often make good use of Gyro Ball if he manages to live long enough to use it. Anchor Shot is a mirror of his Ghost/Grass competitor's signature move, which comes with the amazing utility that is trapping while being a debatably better offensive type than Shackle, but loses out due to Deciduium-Z. Dhelmise also has access to the all-important Rapid Spin, the only Ghost to do so iirc, which comprises the best way to use Dhelmise - an offensive pivot.
Honestly I've never gotten to use a Dhelmise simply due to the late-game nature of the Pokémon. I tend to have full teams of I can by the time I leave Akala - waiting till Poni is just a pain. I considered bringing him on my monograss - aforementioned Steel STAB being the main reason - but I desperately needed something to balance out my weaknesses instead of directly type stacking with my strongest Pokémon.
He's super cool and I love him to death though, like I do most other Grassies.