Tremonius is the first boss you will come across in Halo Infinite. He is a Banished with heavy armor and accompanied by two armored Jackals that can take you down quickly no matter what difficulty you are playing on. Here is how you can beat Tremonius and complete the second level, The Foundation.
After the cutscene introducing Tremonius ends, you do not rush in and fight him up close. He is heavily armored, and the Jackals alongside him will quickly take you down. Instead, when he jumps at you initially, move back and to the right. There will be some spires that will block the Jackals from shooting you and equipment around the area. If you have a Plasma weapon, take down his shields quickly. They are the white bar on his health and do not recharge. After the white bar is gone, switch to a kinetic weapon.
Now that you have created a gap between you and Tremonius, work your way around the pillars and find the Jackals. Taking them out with a grenade is probably the quickest and safest way to dispatch them. They have Manglers, so they can quickly take you down if you get too close. If the grenades are not working, you can also shoot the pink canisters and blow them up nearby, or if you kite them between you and Tremonius, he will shoot them and kill them for you.
This fight gets much easier once the Jackals are out of the way. Just be sure to avoid Tremonius when he lunges at you and take cover when he fires your way. Rely on kinetic weapons and grenades, and he will eventually go down.