In the predatory foram case, you may be shrugging your shoulders and remarking that those forams are pretty damn huge anyway, so it's not that incredible. Alright, I'll grant you that. But what about a fairly small single-celled amoeba tackling nematodes in the soil?
Life cycle of Theratromyxa, involving predation on food a little too large for its size followed by long-term digestion and slumber in cysts. Not a bad lifestyle. (Sayre 1973 J Nematol; Sayre & Wergin 1989 Can J Microbiol)
Imagine you're living your life as a diminutive nematode, and suddenly a small creepy-looking branchy amoeba crawls toward you. Shivers descend down your non-existent spine as the amoeba extends its slender pseudopodia all over your body and gradually engulfs it. Your writhe in terror, but to no avail, for the creepy monster who just moments before appeared tiny and insignificant now has you inside a digestive vacuole full of acid and unfriendly enzymes. If you were lucky, some of your companions were engulfed along with you, so while packed in like sardines, you still have company. You wonder whether this is payback for all the evil you had wrought upon those poor plant roots. Little do you know your entire plight has been carefully planned by your self-proclaimed overlords from another phylum, just to get pretty pictures in the end:
Light micrographs (left; Sayre 1973 J Nematol) and SEM of Theratromyxa (right; Sayre & Wergin 1989 Can J Microbiol). Image 6 shows quite nicely how Theratromyxa captures the nematode. This looks rather similar in principle to the feeding veil of dinoflagellate Protoperidium. Sometimes the amoeba can capture several nematodes at once. SEM shows amoeba enveloping a nematode.
Theratromyxa has been considered for use as a biological control agent for the root-knot nematode (a very tiny group of nematodes, G. Meloidogyne. However, it wasn't particularly effective as excystment was rather slow, and there was no known method of speeding it up. Apparently, anastamosis (joining of numerous pseudopodia/amoebae) has been reported in previous studies, but Sayre 1973 did not observe any. But there still is the possibility of several Theratromyxa individuals (or their relatives) also ganging up on larger prey, as some other protists are known to do (eg. centrohelids cooperating in hunting larger ciliates).
Theratromyxa is a Vampyrellid, a group of rather frightening amoebae, likely in the Endomyxa clade of Cercozoans/Rhizarians (see Pawlowski & Burki 2009 JEM; Parfrey et al. 2010 Syst Biol) (AFAIK, endomyxans are cercozoans, but considering the amount of stuff that's gradually settling in Endomyxa, perhaps the definition of cercozoa is bound to change eventually. I like 'Cercozoa' better than 'Filosea', the other subgroup of cercozoans; ie, it'd be nice to ditch 'Filosea', replace it with 'Cercozoa' and make Endomyxa not Cercozoans. Confused? Don't worry – just taxonomic musings.) Some other Vampyrellids are notorious for poking holes in fungi (Anderson & Patrick 1980 Soil Biol Biochem) and algae (life cycle), and then devouring the cells within. Not a very happy thought if you're a filamentous alga.
By the way, some cercozoan amoeboflagellates can gang up on larger nematodes too, but I'll save that for another day.
Sayre RM (1973). Theratromyxa weberi, An Amoeba Predatory on Plant-Parasitic Nematodes. Journal of nematology, 5 (4), 258-64 PMID: 19319347
Sayre, R., & Wergin, W. (1989). Morphology and fine structure of the trophozoites of Theratromyxa weberi (Protozoa: Vampyrellidae) predacious on soil nematodes Canadian Journal of Microbiology, 35 (5), 589-602 DOI: 10.1139/m89-094